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Experimental study of feeding behavior and interaction of coastal cutthroat trout (Salmo clarki clarki)… Schutz, David C. 1969

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AN EXPERIMENTAL STUDY OF FEEDING BEHAVIOR AND INTERACTION OF COASTAL CUTTHROAT TROUT (Salmo  c l a r k i c l a r k i ) AND DOLLY VARDEN (Salvelinus malma) by DAVID C. SCHUTZ B . S c , U n i v e r s i t y of B r i t i s h Columbia, 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of ZOOLOGY We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA May, 1969 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r a n a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d S t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d b y t h e Head o f my D e p a r t m e n t o r b y h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . The Un i v e r s i t y o f B-V a n c o u v e r 8, C a n a d a D e p a r t m e n t o f i i ABSTRACT Differences i n food habits and s p a t i a l d i s t r i b u t i o n of sympatric Dolly Varden (Salvelinus malma) and cutthroat trout (Salmo c l a r k i c l a r k i ) i n a small coastal lake were documented by Andrusak (MS 1968). Segregation was i n f e r r e d to be of the i n t e r a c t i v e type hypothesized by Nilsson (1965, 1967). The object of t h i s study was to describe feeding behavior of i n d i v i d u a l s from these sympatric populations, and to evaluate the importance of food e x p l o i t a t i o n to the segrega-t i o n process. I n d i v i d u a l and paired f i s h were studied i n the laboratory throughout the spring, summer and autumn. The d i f f e r e n t food habits were found to be due to a number of basic behavioral and morphological differences between the species. Dolly Varden oriented to and rested on the bottom. Cutthroat rested i n the water column and were frequently surface oriented. Searching behavior d i f f e r e d between the species. Dolly Varden swam f a s t e r and at r e l a t i v e -l y constant r a t e s . They sampled "mouthfuls" of substrate as they searched. Trout a l t e r n a t e l y hovered and cruised, sampling s p e c i f i c items. At low l i g h t i n t e n s i t i e s they were much less successful than the char at f i n d i n g benthic food items. The mouth of the Dolly Varden i s small and "scoop-like" compared to that of the cutthroat, and seems p a r t i c u l a r l y well adapted f o r benthic feeding. Dolly Varden searched p e r s i s t e n t l y f o r benthic organisms i n the absence and presence of surface i n s e c t s . Cutthroat r a p i d l y switched from bottom to surface i i i feeding i f i n s e c t s were presented there. The observed differences between species were f u l l y expressed i n i s o l a t e d i n d i v i d u a l s . There was no evidence of the differences being magnified through i n t e r s p e c i f i c competi-t i o n . These d i f f e r e n c e s , believed to be inherent, were considered s u f f i c i e n t to keep the species segregated without the involvement of competition. Segregation was concluded not to be of the interactive, type, even though the populations s t i l l retained considerable p l a s t i c i t y enabling them to switch d i e t s or habitats when necessary or advantageous. The period of intense competition and food e x p l o i t a t i o n was considered to have occurred and ended during e a r l i e r stages of the coexistence. X V TABLE OF CONTENTS Page TITLE PAGE ABSTRACT i i TABLE OF CONTENTS i v LIST OF FIGURES. v i LIST OF TABLES v i i i ACKNOWLEDGEMENTS X INTRODUCTION 1 MATERIALS AND METHODS 3 The F i s h 3 Observations of F i s h i n Marion Lake 6 Holding F a c i l i t i e s i n the Laboratory 7 Experimental F a c i l i t i e s 8 General Laboratory Routine 10 FIELD OBSERVATIONS IN MARION LAKE 11 S p a t i a l D i s t r i b u t i o n 11 Feeding Behavior 14 LABORATORY EXPERIMENTS 16 Sp a t i a l D i s t r i b u t i o n 17 Feeding Behavior 18 1. Learning and prey recognition 18 2. Benthic oriented feeding - i s o l a t e d 23 predators V Page A. Food on substrate surface 2 3 ( i ) Exposed prey 2 3 ( i i ) Camouflaged prey 26 ( i i i ) Searching components 2 7 B. Food within substrate 34 3. Surface oriented feeding - i s o l a t e d 37 predators 4. Surface and benthic feeding - i s o l a t e d 39 predators 5. Int e r a c t i v e feeding - species p a i r s 44 A. Benthic feeding 47 B. Surface feeding 49 C. Benthic and/or surface feeding 51 (i ) Observation tank 51 ( i i ) Outdoor pond 53 6. Light i n t e n s i t y and feeding 54 DISCUSSION 58 SUMMARY 7 6 BIBLIOGRAPHY 78 LIST GF FIGURES FIGURE > Page 1. D i f f e r e n c e s i n mouth s i z e between M a r i o n Lake 5 c u t t h r o a t and D o l l y V arden. 2. Stomach a n a l y s i s o f M a r i o n Lake D o l l y Varden 12 and c u t t h r o a t t r o u t i n a verage p e r c e n t volume, May, A u g u s t , and September 1967. 3. V e r t i c a l d i s t r i b u t i o n o f D o l l y Varden and 13 c u t t h r o a t t r o u t c a u g h t i n 10 m f l o a t i n g n e t s i n M a r i o n Lake May 15 - September 15, 1967. 4. F e e d i n g s u c c e s s o f i n d i v i d u a l e x p e r i e n c e d and 21 n a i v e c u t t h r o a t and D o l l y Varden s e a r c h i n g f o r 50 Chironomus l a r v a e d u r i n g a 30-minute f e e d i n g s e s s i o n . 5. Number o f l a r v a e c a u g h t (10 m i n u t e f e e d i n g 24 s e s s i o n ) on f o u r c o n s e c u t i v e days from a r a t i o n o f 25 g r e e n c h i r o n o m i d s by i n d i v i d u a l D o l l y Varden and c u t t h r o a t t r o u t . 6. T y p i c a l s e a r c h i n g p a t t e r n o f c u t t h r o a t and D o l l y 29 Varden l o o k i n g f o r c h i r o n o m i d l a r v a e i n a sand and l e a f - l i t t e r s u b s t r a t e . 7. Average t i m e i n seconds t a k e n by D o l l y Varden 40 and c u t t h r o a t t o c a p t u r e s i n g l e D r o s o p h i l a a d u l t s d u r i n g t h e f i r s t f o u r days t h a t f l i e s were p r e s e n t e d . 8. Number o f t r i p s t o t h e t a n k bottom and average 41 t i m e i n seconds o f each by c u t t h r o a t and D o l l y Varden d u r i n g t h e i r f i r s t f o u r f e e d i n g s e s s i o n s on D r o s o p h i l a a d u l t s . 9« C u m u l a t i v e " s u r f a c e g r a b s " by i n d i v i d u a l c u t t h r o a t 43 and D o l l y Varden f e e d i n g on a r a t i o n o f 50 Chironomus 1ar v a e and 25 D r o s o p h i l a a d u l t s . 10. C o m p a r a t i v e s u c c e s s o f p a i r e d D o l l y Varden and 48 c u t t h r o a t " s c r a m b l i n g " f o r 20 f o o d i t e m s i n t r o d u c e d - o n e a t a t i m e a t t h e b ottom. 11. C o m p a r a t i v e s u c c e s s o f p a i r e d D o l l y Varden and 50 c u t t h r o a t " s c r a m b l i n g " f o r 20 f o o d i t e m s introduced..,one a t a t i m e a t t h e s u r f a c e . FIGURE 12. Comparative success of paired Dolly Varden and cutthroat "scrambling" f o r 20 food items introduced... simultaneously at the surface and bottom. 13. Mean number of larvae caught from a r a t i o n of 25 Chironomus by i n d i v i d u a l Dolly Varden and cutthroat during a 10-minute feeding session. A = water surface l i g h t i n t e n s i t y , B = 48 cm below surface or 8 cm above bottom. v i i i L IST OF TABLES TABLE Page I L e n g t h and w e i g h t o f e x p e r i m e n t a l f i s h . 6 I I Depth a t w h i c h f i s h were o b s e r v e d i n - 14 M a r i o n L a k e , August and September 1968. I I I F e e d i n g l o c a t i o n s o f D o l l y Varden and 15 c u t t h r o a t o b s e r v e d i n M a r i o n L a k e , September 5, and 6, 1968. I V S i z e d i f f e r e n c e s o f D o l l y Varden seen n e a r 15 t h e bottom and i n t h e w a t e r column o f M a r i o n L a k e , August and September 1968. V Numbers^ o f f i s h seen t o g e t h e r i n M a r i o n L a k e , 16 A u g u s t and September 1968. VI " R e s t i n g " p o s i t i o n s o f 10 c u t t h r o a t and 18 10 D o l l y Varden i n o b s e r v a t i o n t a n k s . V I I F e e d i n g s u c c e s s o f i n d i v i d u a l c u t t h r o a t 25 and D o l l y Varden s e a r c h i n g f o r 50 Chironomus l a r v a e on a sand and l e a f - l i t t e r s u b s t r a t e d u r i n g a 30-minute f e e d i n g s e s s i o n . V I I I F e e d i n g s u c c e s s o f i n d i v i d u a l c u t t h r o a t and 28 D o l l y Varden s e a r c h i n g f o r 25 g r e e n c h i r o n o m i d l a r v a e on a sand and l e a f - l i t t e r s u b s t r a t e d u r i n g a 10-minute f e e d i n g s e s s i o n . IX A v e rage swimming speeds o f i n d i v i d u a l 30 c u t t h r o a t and D o l l y Varden s e a r c h i n g f o r Chironomus l a r v a e on a sand and l e a f - l i t t e r s u b s t r a t e . XI Number o f bottom g r a b s by i n d i v i d u a l c u t t h r o a t 33 and D o l l y Varden d u r i n g a 10-minute f e e d i n g s e s s i o n on Chironomus l a r v a e . X I S t r i k e s u c c e s s o f i n d i v i d u a l c u t t h r o a t and. 34 D o l l y Varden f e e d i n g on r e d Chironomus l a r v a e on a sandy s u b s t r a t e . X I I F e e d i n g s u c c e s s o f i n d i v i d u a l c u t t h r o a t and 36 D o l l y Varden s e a r c h i n g f o r T u b i f e x i n a sand s u b s t r a t e d u r i n g a 10-minute f e e d i n g s e s s i o n . i x TABLE XIII XIV XV Time taken by i n d i v i d u a l cutthroat and Dolly Varden to pick up and consume 2 5 Tubifex on the bottom when no substrate present. Comparative feeding success during a 30 minute session, of i n d i v i d u a l cutthroat and Dolly Varden searching f o r 50 Chironomus larvae i n the presence and absence of a surface r a t i o n of 25 Drosophila. Stomach contents of Dolly Varden and cutthroat held together f o r previous 24 days i n outdoor pond. Page 37 45 55 X ACKNOWLEDGEMENTS F i n a n c i a l support f o r t h i s study came from the B. C. Government F i s h and W i l d l i f e Branch, National Research Council, and U n i v e r s i t y of B r i t i s h Columbia I n s t i t u t e of F i s h e r i e s . The guidance and enthusiasm of my supervisor, Dr. T. G. Northcote, were greatly appreciated throughout the study. I would l i k e to thank Dr. J . D. McPhail f o r his i n t e r e s t and active assistance, p a r t i c u l a r l y during the period when my supervisor was away. Mr. J . Bryan helped to analyse the data. Drs. G. Scudder, R. L i l e y , and N. Nilsson, read the manuscript and offered many useful suggestions. My thanks go to Mr. H. Andrusak for h i s companionship and assistance during the f i e l d work. Many other i n d i v i d u a l s from the I n s t i t u t e of F i s h e r i e s also deserve thanks. Special thanks go to my wife, her parents, and mine f o r t h e i r endless patience, assistance, and encouragement. Mrs. D. McQueen typed the t h e s i s , and Mrs. L. L i c h t prepared the f i g u r e s . 1 INTRODUCTION Throughout northern temperate regions there are numerous examples of coexisting c l o s e l y r e l a t e d f i s h e s . Many of these are salmonids, and examples are described by Kalleberg ( 1 9 5 8 ) , Nilsson ( 1 9 6 5 ) , Newman ( 1 9 5 6 ) , Everhart and Waters ( 1 9 6 5 ) , and Hartman ( 1 9 6 5 ) . A l l s-almonid species are s i m i l a r i n morphology and trophic adaptations, and are there-fore p o t e n t i a l competitors. Nilsson (1967) points out that though i t i s often easy to demonstrate segregation of coexisting f i s h e s i t i s d i f f i c u l t to determine i f the segregation i s i n t e r a c t i v e or due to completed e c o l o g i c a l divergence. Nilsson (1955, 60, 61, 63, 65, 67) has looked i n d e t a i l at coexi s t i n g populations of Salvelinus alpinus and Salmo t r u t t a i n Sweden, and has develop-ed the hypothesis that t h e i r co-existence i s due to i n t e r a c t i v e segregation. He suggests that most species of f i s h i n northern temperate regions may provide examples of i n t e r a c t i v e segrega-t i o n since the faunas are young and the ecosystems are s t i l l evolving r e l a t i v e l y r a p i d l y . The term, i n t e r a c t i v e segregation, was o r i g i n a l l y used by Brian ( 1 9 5 6 ) , and means that any e c o l o g i c a l d i f f e r e n c e s , such as food or habitat s e l e c t i o n , between the species are enlarged considerably when the species are sympatric. Nilsson (1967) suggested the following mechanisms are possibly involved i n i n t e r a c t i v e segregation: a. E x p l o i t a t i o n - "one species i s more e f f i c i e n t i n a habitat than another, perhaps because i t can f i n d and use v a c a n t r e s o u r c e s more q u i c k l y t h a n t h e o t h e r " ( B r i a n 1956). b. T e r r i t o r i a l i t y c. Pood f i g h t i n g d. P r e d a t i o n e. O t h e r i n t e r f e r e n c e s . A second t y p e o f s e g r e g a t i o n d e f i n e d by B r i a n i s s e l e c t i v e s e g r e g a t i o n . T h i s t y p e he says " c o n s i s t s o f t h e s e l e c t i o n by i n s t i n c t i v e b e h a v i o r o f t o t a l l y d i f f e r e n t h a b i t a t ( t h a t i s r e s o u r c e s and m o d i f y i n g f a c t o r s c o m b i n e d ) " . N i l s s o n (1965) m e n t i o n s t h a t t h e work o f F r y e r (1959) on Lake Nyasa c i c h l i d s i n d i c a t e s - s e l e c t i v e s e g r e g a t i o n . S e l e c t i v e s e g r e g a — t i o n c o u l d t h e o r e t i c a l l y be an end r e s u l t w i t h i n t e r a c t i v e s e g r e g a t i o n an i n t e r m e d i a t e s t e p . I n B r i t i s h C o l u m b i a Andrusak (MS 1968) s t u d i e d a l l o -p a t r i c and s y m p a t r i c p o p u l a t i o n s o f c u t t h r o a t t r o u t (Salmo c l a r k i c l a r k i ) and D o l l y Varden ( S a l v e l i n u s malma) i n t h r e e s m a l l c o a s t a l l a k e s . He d e s c r i b e d d i s t i n c t s p a t i a l and f o o d s e g r e g a t i o n s d u r i n g t h e summer f o r t h e s y m p a t r i c p o p u l a -t i o n s i n M a r i o n Lake w h i c h s u g g e s t s i n t e r a c t i v e s e g r e g a t i o n i s p r e s e n t between t h e s e s p e c i e s i n much t h e same manner as d e s c r i b e d by N i l s s o n (1967) f o r S a l v e l i n u s a l p i n u s and Salmo t r u t t a . A n d r u s a k ' s f i e l d s t u d y c l e a r l y d e m o n s t r a t e s a season a l s e g r e g a t i o n o f t h e two s p e c i e s i n M a r i o n L a k e . The s t u d y 3 did not include any de t a i l e d i n v e s t i g a t i o n of mechanisms i n -volved i n the segregation process. The objectives of my study were twofold: 1. To describe i n d e t a i l the feeding behavior of lake dwelling coastal cutthroat and Dolly Varden. 2. To look s p e c i f i c a l l y at food e x p l o i t a t i o n as a mechanism f o r segregation; v i z , to determine i f there were differences i n the feeding behavior of the two species, that could contribute s i g -n i f i c a n t l y to t h e i r segregation. Based on f i e l d observations and Andrusak's e a r l i e r study i t was hypothesized that Dolly Varden were more success-f u l predators than cutthroat on benthic organisms, and that cutthroat were more successful than Dolly Varden feeding on surface organisms. Experiments were then conducted to tes t these hypotheses. Brian's (1956) d e f i n i t i o n of the e x p l o i t a t i o n mechanism states that one species i s more e f f i c i e n t than another, but does not define what i s meant by e f f i c i e n t . In t h i s study the term, more e f f i c i e n t , means that one species caught s i g n i f i c a n t l y more food than the other i n a given time. No attempt was made to r e l a t e energy obtained to energy expended. MATERIAL AND METHODS THE FISH Experimental f i s h used i n the study were mature 4 coasta l cutthroat (Salmo c l a r k i c l a r k i ) , and coastal Dolly Varden (Salvelinus malma). Cutthroat were t y p i c a l coastal specimens as des-crib e d by C a r l , Clemens, and Lindsey (1959) or Clemens and Wilby (1961). The jaw was long, extending well beyond the po s t e r i o r margin of the eye, and the snout was r e l a t i v e l y pointed. Dolly Varden from Marion Lake d i f f e r e d considerably from the general d e s c r i p t i o n of the species given by the above authors. Clemens and Wilby (1961) say that the head and mouth are large with the maxillary reaching to a point behind the po s t e r i o r margin of the eye. This d e s c r i p t i o n i s very s i m i l a r to that of the cutthroat's head. Marion Lake Dolly Varden had small heads and mouths compared to Marion Lake cutthroat ( F i g . 1). The mouth was nearly subterminal and the snout blunt, whereas the cutthroat's mouth was d i s t i n c t l y terminal and the snout pointed. Marion Lake f i s h were used e x c l u s i v e l y f o r a l l experiments except those i n v o l v i n g paired cutthroat and Dolly Varden. A, few i n d i v i d u a l s i n t h i s s e r i e s came from other c o a s t a l lakes.. A l l cutthroat were angled and a l l Dolly Varden g i l l netted. Most of the data came from four cutthroat and four Dolly Varden from Marion Lake. These w i l l be re f e r r e d to as CT #1, #2, #3, #4, and DV #1, #2, #3, and #4. A l l were approximately equal i n s i z e (Table I ) . Fis h used i n the study a l l appeared to be i n good F O R K L E N G T H (mm) F I G . 1 D i f f e r e n c e s i n mouth s i z e b e t w e e n M a r i o n L a k e c u t t h r o a t and D o l l y V a r d e n 6 TABLE I . L e n g t h and w e i g h t o f e x p e r i m e n t a l f i s h . Weight (g) L e n g t h (mm) Date o f l e n g t h -w e i g h t measurements CT #1 97.5 212 December/68 CT #2 57.5 171 October/68 * CT #3 54.2 174 December/68 CT #4 64.5 185 December/68 D.V #1 79.3 192 December/68 DV #2 64.6 182 October/68 * DV #3 69.8 182 December/68 DV #4 70.0 179 December/68 * F i s h d i e d i n O c t o b e r c o n d i t i o n . I f an i n d i v i d u a l d i d n o t seem h e a l t h y t h e n i t was n o t u s e d . C u t t h r o a t #2 and D o l l y Varden #2 d i e d s u d d e n l y from unknown c a u s e s i n O c t o b e r 1968. O t h e r w i s e a l l f i s h seemed t o grow and s u r v i v e w e l l d u r i n g t h e e x p e r i m e n t s . CT #1 g a i n e d 39 g and grew 32 mm between A p r i l 1, 1968 and December 15, 1968. DV #1 g a i n e d 33 g and grew 2 7 mm i n t h e same p e r i o d . D u r i n g t h i s t i m e t h e f i s h were f e d an e x c e s s o f f r o z e n b r i n e s h r i m p each e v e n i n g , b u t t h e i r stomachs were f r e q u e n t l y e v a c u a t e d t h e f o l l o w i n g m o r n i n g . OBSERVATIONS OF FISH IN MARION LAKE Most o b s e r v a t i o n s were made from a row b o a t w i t h t h e 7 a i d o f a p l e x i g l a s s v i e w i n g box. Two p e r s o n s were i n t h e b o a t , one o b s e r v i n g , t h e o t h e r s t e e r i n g w i t h o a r s and r e c o r d -i n g d a t a . I t was seldom n e c e s s a r y t o row s i n c e p r e v a i l i n g b r e e z e s moved t h e bo a t s u f f i c i e n t l y . The v i e w i n g box was moved s l o w l y from s i d e t o s i d e b e h i n d t h e transom t o s c a n as wide a f i e l d as p o s s i b l e . A few f i s h s i g h t i n g s were made from t h e b o a t t h r o u g h p o l a r o i d sun g l a s s e s , and o t h e r s by swimming a l o n g t h e s u r f a c e w i t h mask and s n o r k e l . S i g h t i n g s were r e s t r i c t e d a l m o s t e x c l u s i v e l y t o one 100-meter s e c t i o n o f t h e l a k e , p a r a l l e l t o t h e s h o r e and i n -c l u d i n g d e p t h s t o 4 m e t e r s . T h i s r e g i o n had a g e n t l y s l o p i n g mud b o t t o m , many r o o t e d a q u a t i c s , and submerged l o g s . A l l o b s e r v a t i o n s were made between 0900 and 1500 h o u r s s i n c e shadows from t h e s u r r o u n d i n g mountains made s p o t t i n g and i d e n t i f i c a t i o n d i f f i c u l t b e f o r e and a f t e r t h i s p e r i o d . HOLDING F A C I L I T I E S IN THE LABORATORY F i s h were h e l d i n d o o r s i n two c o n c r e t e t r o u g h s 366 cm x 41 cm x 38 cm. D e n s i t i e s v a r i e d f r om 20 t o 30 f i s h p e r t r o u g h and i n c l u d e d b o t h s p e c i e s . A, s t e a d y f l o w o f w a t e r was m a i n t a i n e d i n t h e h o l d i n g t a n k s w i t h c o m p l e t e exchange t a k i n g about t h r e e h o u r s . Water t e m p e r a t u r e was- u s u a l l y about 1 C h i g h e r t h a n t h e i n c o m i n g c i t y w a t e r , and ranged from 6 C i n J a n u a r y t o 14 C i n A u g u s t . E h o t o p e r i o d i n t h e h o l d i n g a r e a was n a t u r a l , t h e o v e r h e a d f l u o r e s c e n t f i x t u r e s b e i n g c o n t r o l l e d by a p h o t o -8 e l e c t r i c c e l l located outside. EXPERIMENTAL FACILITIES IN THE LABORATORY Observation Tanks The four experimental tanks were housed i n a 3.3 m x 4.5 m room where noise and interference were n e g l i g i b l e . The tanks 122 cm x 61 cm x 61 cm were made of painted plywood with p l e x i g l a s s f r o n t s . Three of them were subdivided i n the middle by removable f i b e r g l a s s screen d i v i d e r s . A Dolly Varden was held i n one end and cutthroat i n the other. Screened l i d s kept the f i s h from jumping out. The fourth tank was not divided and was used f o r the d a i l y experiments. V e r t i c a l l i n e s on the plexiglass: front defined four equal quadrants. Each tank was illuminated by a si n g l e "cool white" General E l e c t r i c fluorescent lamp providing surface i l l u m i n a -t i o n of approximately 50G lux. Illumination at a depth of 48 cm (8 cm from the tank bottom) was approximately 240 lux. This was as close to the bottom as the phototube could be placed. A l l feeding experiments, unless stated otherwise, were, at t h i s l i g h t i n t e n s i t y . Photoperiod i n the observation room consisted of 14 hours "days" and 10 hours "nights" f o r the duration of the study. Black p l a s t i c curtains were hung between and above the tanks to keep the room r e l a t i v e l y dark. Observations were made from a cha i r 1 to 3 meters i n front of the tank. Water temperature and flow were s i m i l a r to conditions described f o r the concrete holding tanks. 9 R e c o r d i n g Equipment B e h a v i o r was r e c o r d e d w i t h t h e a i d o f a t a p e r e c o r d e r , e i g h t k e y m u l t i p l e c o u n t e r , and s t o p w a t c h. I l l u m i n a t i o n l e v e l s were v a r i e d by r e p l a c i n g t h e f l u o r e s c e n t lamp w i t h two 60 w a t t "Shadow Ban" w h i t e i n c a n -d e s c e n t b u l b s i n c i r c u i t w i t h a V a r i a c v o l t a g e r e g u l a t o r . L i g h t i n t e n s i t i e s were measured w i t h a P h o t o v o l t Model 514 M photometer f i t t e d w i t h p h o t o t u b e C and a s c o t o p i c (human eye) c o r r e c t i o n f i l t e r . The p h o t o t u b e was c a l i b r a t e d i n June 1968 u s i n g a N.B.S. f o o t c a n d l e lamp. I t was housed i n an u n d e r w a t e r c a s e w i t h an o p a l d i f f u s i n g g l a s s p r o d u c i n g a r e a s o n a b l y c l o s e f i t t o a c o s i n e r e s p o n s e c u r v e . By c o n t r o l l i n g i l l u m i n a t i o n l e v e l s w i t h t h e V a r i a c , s p e c t r a l c h a r a c t e r i s t i c s o f t h e l i g h t were a l t e r e d c o n s i d e r a b l y , t h e r e d s p r e d o m i n a t i n g a t t h e l o w e s t i n t e n s i t i e s . Readings i n f o o t - c a n d l e o r l u x u n i t s were t h e r e f o r e v e r y a p p r o x i m a t e . Stomach Pump Stomachs were e v a c u a t e d w i t h a stomach s a m p l e r s i m i l a r t o t h a t d e s c r i b e d by Seaburg ( 1 9 5 7 ) . The sa m p l e r u s e d had a 1.5 mm i n s i d e d i a m e t e r c o p p e r i n l e t t u b e and 6 mm I . E . o u t l e t t u b e . Both t u b e s were 140 mm l o n g . The pump was 100% e f f i c i e n t i n r e c o v e r i n g c h i r o n o m i d l a r v a e e a t e n i n t h e p r e c e d i n g h o u r . T h i s was e s t a b l i s h e d from numerous t r i a l s i n wh i c h l a r v a e i n t h e tan k were c o u n t e d b e f o r e and a f t e r a f e e d i n g s e s s i o n , and t h e d i f f e r e n c e s c h e c k e d a g a i n s t t h e 10 number e v a c u a t e d f rom t h e stomach. E i s h were a n a e s t h e t i z e d i n a .06% s o l u t i o n o f 2 - p h e n o x y e t h a n o l p r i o r t o pumping. No a d v e r s e e f f e c t s from t h i s p r o c e d u r e were e v i d e n t . G.T #1 had i t s : s±omach e v a c u a t e d a p p r o x i m a t e l y 85 t i m e s between June 20 and November 30, 1968. B e h a v i o r and c o n d i t i o n o f t h i s f i s h a p p e a r e d " n o r m a l " f o r t h e d u r a t i o n . GENERAL LABORATORY ROUTINE D o l l y Varden and c u t t h r o a t i n use were h e l d i n t h r e e o f t h e f o u r a q u a r i a i n t h e o b s e r v a t i o n room. A l l e x p e r i m e n t s i n v o l v i n g s i n g l e f i s h were c o n d u c t e d i n t h e f o u r t h t a n k , t h e e x p e r i m e n t a l f i s h b e i n g l i f t e d i n t o t h e tan k i m m e d i a t e l y p r i o r t o t h e e x p e r i m e n t and removed i m m e d i a t e l y f o l l o w i n g i t . E x p e r i m e n t a l f i s h i n t h e o b s e r v a t i o n room u s u a l l y had t h e i r stomachs e v a c u a t e d i n t h e morning i f e x p e r i m e n t s were s c h e d u l e d f o r l a t e r i n t h e day. T h i s was done i n an a t t e m p t t o s t a n d a r d i z e hunger l e v e l s . No e x p e r i m e n t s were r u n u n t i l a t l e a s t an ho u r a f t e r e v a c u a t i o n , a l t h o u g h t h e f i s h w o uld f e e d w i t h i n 5 t o 10 m i n u t e s o f r e c o v e r y from t h e a n a e s t h e t i c . The stomach pump was a l s o used a t t h e end o f each f e e d i n g e x p e r i m e n t t o g e t a d i r e c t c o u n t o f t h e number o f i n v e r t e b r a t e s c a p t u r e d . E i s h i n t h e h o l d i n g t r o u g h s were f e d d a i l y . T h e i r f o o d a l t e r n a t e d between f r o z e n b r i n e s h r i m p , f r o z e n f i s h , and h o r s e h e a r t . 11 FIELD OBSERVATIONS IN MARION LAKE Results of g i l l netting and stomach analysis by Andrusak (MS 1968) indicated that during July and August > Marion Lake Dolly Varden fed predominantly on the benthos and cutthroat at the surface ( F i g . 2). F i e l d studies were con-ducted to t r y to substantiate Andrusak's findings by d i r e c t observation. SPATIAL DISTRIBUTION A l l observations were r e s t r i c t e d to depths of 4 meters or less (Table I I ) . Within t h i s zone 40% of the Dolly Varden observed were swimming within a few centimeters of the bottom and 60% were i n mid-water. The absence of r e s t i n g f i s h may have been p a r t i a l l y due to the d i f f i c u l t y of s:eeing immobile f i s h l y i n g on the bottom. No Dolly Varden were seen between the shore and the 2-meter contour. A l l cutthroat seen were within 1 to 2 meters of the surface. F i f t y f i v e per cent of these were i n s i d e the 2-meter depth contour, but none was c l o s e l y associated with the bottom. Cutthroat were frequently hovering i n the shallows when f i r s t spotted, but within seconds moved away. They seemed more wary than the Dolly Varden who could often be watched f o r several minutes before being disturbed. The observed v e r t i c a l d i s t r i b u t i o n corresponded c l o s e l y with Andrusak's g i l l net r e s u l t s ( F i g . 3). 12 DOLLY VARDEN CUTTHROAT MAY 14 MAY 23 AUGUST SEPTEMBER FIG. 2 Stomach analysis of Marion Lake Dolly Varden and cutthroat trout i n average per cent volume, May, August, and September, 1967 (adapted from F i g . 17, Andrusak 1968 MSc The s i s ) . 13 Or to a Id p-UJ 2 X a 6| UJ a 5 F I SH lO1-DOLLY VARDEN N=362 CUTTHROAT N-II2 FIG. 3 V e r t i c a l d i s t r i b u t i o n of Dolly Varden and cutthroat trout caught i n 10 rn f l o a t i n g nets i n Marion Lake, May 15 - Sept. 15, 1967 (from F i g . 12, Andrusak 1968 MSc T h e s i s ) . 14 TABLE I I . Depth at which f i s h were observed i n Marion Lake, August and September 1968. 0 meters to 2 meters 2 meters to 4 meters Bottom Midwater Bottom Midwater Cutthroat 0 14 0 11 Dolly V.arden 0 0 30 48 FEEDING BEHAVIOR Dolly Varden were frequently observed feeding on the benthos and i n the water column i n Marion Lake ( T c i b l e I I I ) . Feeding cutthroat were only p o s i t i v e l y i d e n t i f i e d two or three times and a l l were taking surface i n s e c t s . I t seems l i k e l y that the reason more feeding trout were not seen i s that they were close to the surface and therefore frightened by the boat before being observed. Probably the majority of f i s h surfacing i n the lake were cutthroat. During the summers of 1967 and 1968 approximately 75 were angled with worms, a r t i f i c i a l f l i e s , or spinners within a meter of the surface. Only three Dolly Varden were caught i n t h i s manner. Dolly Varden feeding on the benthos swam slowly, p a r a l l e l to the bottom, with frequent momentary stops as they dipped to pick up and then r e j e c t bottom materials. Those feeding on plankton constantly changed d i r e c t i o n both l a t e r a l l y and v e r t i c a l l y as they "mouthed" at objects i n the water column. 15 TABLE. I I I . Feeding locations of Dolly Varden and cutthroat observed i n Marion Lake, September 5 and 6, 1968. Feeding on Feeding on Angled with benthos midwater f l y at plankton surface Cutthroat 0 0 20 Dolly Varden 13 12 0 Sixty f i v e of the 78 Dolly Varden observed were small (100 - 150 mm). Andrusak (MS 1968) noted that Dolly Varden caught during the summer i n three meters or less were often small. Forty s i x of the 65 Dolly Varden i n t h i s small s i z e c l a s s were seen i n mid water whereas 11 of 13 i n the 150 - 200 mm siz e class were very close to the bottom (Table IV). This corresponds with Andrusak's findings that the smaller Dolly Varden fed heavily on plankton i n late summer whereas the 100 - 200 mm group fed p r i m a r i l y on bottom fauna. TABLE IV. Size differences of Dolly Varden seen near the bottom and i n the water column of Marion Lake, August and September 1968. Bottom Midwater Small (<150 mm) Large ( >150 mm) 19 11 46 2 16 D o l l y Varden appeared t o be much more g r e g a r i o u s t h a n c u t t h r o a t . B o t t o m - f e e d i n g i n d i v i d u a l s r e p e a t e d l y were seen t r a v e l l i n g i n twos and t h r e e s and p l a n k t o n f e e d e r s were seen i n l o o s e a g g r e g a t e s o f up t o s i x o r more ( T a b l e V ) . Minimum d i s t a n c e s between i n d i v i d u a l s were e s t i m a t e d t o be about 10 cm. Two p a i r s o f c u t t h r o a t were o b s e r v e d . The o t h e r s were a l l s i n g l e s . A n d r u s a k ' s g i l l n e t d a t a showed t h a t D o l l y Varden were h i g h l y clumped d u r i n g t h e summer, and c u t t h r o a t d i s t r i b u t i o n s were e i t h e r d i s p e r s e d o r random. TABLE V. Numbers o f f i s h seen t o g e t h e r i n M a r i o n L a k e , August and September .1968. S i n g l e s Twosr Threes F o u r s F i v e s S i x e s > S i x C u t t h r o a t 23 2 0 0 0 0 0 D o l l y Varden 16 6 6 1 0 2 1 Combined c u t t h r o a t and D o l l y Varden 0 0 0 0 0 0 LABORATORY EXPERIMENTS To t e s t t h e h y p o t h e s e s t h a t D o l l y Varden f e e d more s u c c e s s f u l l y t h a n c u t t h r o a t on b e n t h i c o r g a n i s m s , and t h a t c u t t h r o a t a r e t h e more s u c c e s s f u l s p e c i e s a t t h e s u r f a c e , a s e r i e s o f l a b o r a t o r y e x p e r i m e n t s were c o n d u c t e d . These e x p e r i m e n t s were d e s i g n e d t o d e t e r m i n e s p a t i a l p r e f e r e n c e s , and t o compare t h e r a t e o f l e a r n i n g and f e e d i n g e f f i c i e n c y o f 17 i n d i v i d u a l s from each species when offered surface organisms only, bottom organisms only, or a combination of both. SPATIAL DISTRIBUTION Dolly Varden and cutthroat i n the laboratory d i f f e r e d considerably i n t h e i r r e s t i n g habits. These differences were qu a n t i f i e d by watching i s o l a t e d i n d i v i d u a l s of each species p e r i o d i c a l l y throughout the day, and recording the time spent at each l o c a t i o n . Between feedings Dolly Varden (1 per tank) spent an average of 50 to 75% of t h e i r time l y i n g on the tank bottom. I t was uncommon to see them r e s t i n g anywhere else (Table VI). This behavior seems t y p i c a l of the genus Sa l v e l i n u s . Newman (1960) described i t f o r Dolly Varden, Nilsson (pers. comm.) fo r a r c t i c char (S. alpin u s ) , and Newman (1956) f o r brooktrout (S. f o n t i n a l i s ) . Resting Dolly Varden sometimes lay on t h e i r b e l l i e s , but frequently the body was held o f f the bottom by the caudal, p e l v i c , and pectoral f i n s . When not r e s t i n g Dolly Varden usually swam vigorously around the tank. This swimming was frequently up and down i n the tank corners and back and f o r t h against the pl e x i g l a s s f r o n t . I t was considered to be a type of escape a c t i v i t y and pe r s i s t e d i n t e r m i t t e n t l y i n some f i s h f o r months. Isolated cutthroat i n aquaria generally hovered i n one place i n the water column f o r most of the time. I t was d i f f i c u l t to t e l l i f hovering f i s h f i x e d t h e i r p o sitions i n 18 r e l a t i o n to distance o f f the bottom or from the surface. Some hovered j u s t beneath the surface while others stayed within 2 or 3 cm of the bottom. In e i t h e r case they were never more than 50 cm from the air-water i n t e r f a c e , and most seemed to be v i s u a l l y oriented to the surface. TABLE VI. "Resting" positions of 10 cutthroat and 10 Dolly Varden i n observation tanks (single f i s h i n tanks; data represent one hour of " r e s t i n g " time per f i s h ) . Bottom Distance above the bottom 2-10 cm 11-20 cm 21-• 30 cm 31-40 cm Cutthroat 1 3 2 3 1 Dolly Varden 9 0 0 1 0 Trout and char i n the holding troughs were held to-gether i n mixed groups. The troughs were not su i t a b l e f o r lengthy observation work, but usually one could get a b r i e f look at the f i s h before fr i g h t e n i n g them. Most of the Dolly Varden lay on the bottom and a few were always c r u i s i n g or hovering, but a l l of the cutthroat hovered o f f the bottom. FEEDING BEHAVIOR 1. Learning and prey recognition During the f i r s t two weeks i n the laboratory when experimental techniques were being improved, and p i l o t exper-iments t r i e d , i t became apparent that the Dolly Varden and 19 cutthroat i n use (DV #1.and CT #1) improved i n t h e i r a b i l i t y to capture chironomid larvae f o r four or f i v e consecutive days. On each of these days the f i s h spent one 30 to 40 minute period searching f o r the larvae. Thereafter, a d d i t i o n a l experience no longer resulted i n increased captures per unit of time. The number of larvae caught i n subsequent half-hour feeding sessions l e v e l l e d o f f with s l i g h t d a i l y v a r i a t i o n i n e i t h e r d i r e c t i o n . An experiment was subsequently set up to compare the capturing a b i l i t i e s of "naive" and "experienced" f i s h of both species searching f o r chironomids. Naive f i s h were those that had been captured at Marion Lake 9 months e a r l i e r , and had at no time during these months been exposed to chironomid larvae. These f i s h were considered to be naive f o r only one feeding session on the larvae. Experienced f i s h were those that had previously reached a plateau i n larvae captured. Indiv i d u a l f i s h were given 30-minute feeding sessions on 50 red Chironomus sp. larvae i n a sand and " l e a f - l i t t e r " substrate. The substrate was a 5 mm deep mixture of f i v e parts sand under one part l e a f - l i t t e r . L e a f - l i t t e r consisted of the evergreen needle and twig residue l e f t a f t e r sieving Marion Lake bottom samples. Larvae were d i s t r i b u t e d i n the tank 15 minutes before 2 introducing the f i s h , at an average density of 1 per 132 cm . Approximately 10% buried themselves completely within minutes, but the majority remained exposed f o r the duration of the experiment. 20 Each f i s h was l i f t e d i n a dip net from i t s holding tank i n t o the observation tank, and the feeding session was considered to have started when i t made i t s f i r s t feeding i n t e n t i o n movement. A't the end of 30 minutes i t was removed and anaesthetized. Captured larvae were recovered with the stomach pump. Experienced cutthroat and Dolly Varden began to search within a minute or two of being placed i n the tank. Naive f i s h were held i n the observation room f o r only 24 hours p r i o r to the experiment and many would not eat a f t e r being moved from one tank to another. To be sure that the naive experimental f i s h would eat, each was offered a small portion of brine shrimp at the s t a r t of the experiment. Those that refused the shrimp- were not used. During each feeding period the following elements of behavior were recorded: 1. "Bottom grabs" (B.G.) - included a l l attempts by the f i s h to pick up something from the bottom. 2. Quadrant changes - scored each time the fishtmoved from one to another of four equal sections i n the tank. The four naive cutthroat and f i v e naive Dolly Varden captured an average of 13% and 12% re s p e c t i v e l y of the 50-chironomid r a t i o n . Experienced trout and char a l l scored between 25% and 30% ( F i g . 4). Naive f i s h of both species made 21 55-5 0 -o Hi 45-cr r- 4 0 -a < u 35-LU < > Ct 3 0 -< _l ft 25-H Z UJ 2 0 -O c£ H! 15-I O -5-O -CUTTHROAT OC55 j(2) C4> 6(4) DOLLY VARDEN 1 > i f 5 ) , L C 5 ) CT«I CT#2 -EXPERIENCED n JL — J 1—4 NAIVE — J >— DV#I D V * 2 E X P E R I E N C E D - 1 ' 55 -50 - 4 5 -AO - 3 5 - 3 0 - 2 5 -20 -15 - I O -5 -O 5 N A I V E - 1 10 </) Hi o u z u cc Ui a F I G . 4 F e e d i n g s u c c e s s o f i n d i v i d u a l e x p e r i e n c e d and n a i v e c u t t h r o a t and D o l l y Varden s e a r c h i n g f o r 50 Chironomus l a r v a e d u r i n g a 30 min u t e f e e d i n g s e s s i o n . Mean p e r c e n t o f l a r v a l r a t i o n c a p t u r e d = s o l i d s ymbols. Mean p e r c e n t s u c c e s s (open symbols) = no. o f c a p t u r e s no. o f bottom g r a b s . V e r t i c a l l i n e s i n d i c a t e r a n g e ; number o f r e p l i c a t e s i n b r a c k e t s . 22 proportionately more grabs at non-living objects than did the experienced f i s h . Per cent success was therefore lower f o r the naive than experienced predators. Naive f i s h did not seem to recognize larvae as r e a d i l y as experienced ones. One cut-throat and one Dolly Varden captured as many larvae as the most successful experienced f i s h , but per cent success was only 14% and 11% compared to a mean of 25% or more fo r each of the experienced predators. These high capture scores were due to the f i s h sampling an exceptionally large amount of needles, s t i c k s , and bottom debris. The r e s u l t s indicated that cut-throat and Dolly Varden have to learn to feed e f f e c t i v e l y on new types of prey. No differences were apparent between the trout and char. A temporary shortage of red Chironomus larvae led to the next experiment. During a feeding session with one of the experienced Dolly Varden a 40-larvae r a t i o n was made up of ten Ghironomus and 30 green Psectrotanypus. At the end of the period evacuation of the stomach revealed four Chironomus only. Before t h i s observation i t was assumed that once a f i s h commenced feeding on chironomid larvae, a l l exposed species would be vulnerable. To t e s t t h i s assumption four cutthroat and Dolly Varden, experienced feeders on red Chironomus, were each given 10-minute feeding sessions on green Psectrotanypus on four consecutive days. Larvae were d i s t r i b u t e d on a sand substrate with no l e a f - l i t t e r . These larvae did not burrow i n the sand. The four predators had demonstrated previously that within 23 10 m i n u t e s each would p i c k up 20 o r more r e d Ghironomus l a r v a e f r o m an u n b u r i e d r a t i o n o f 25. I n t h e f i r s t t e s t , t h r e e o f t h e f o u r f i s h c a p t u r e d no l a r v a e i n a t e n m i n u t e f e e d i n g s e s s i o n ; one t o o k s i x l a r v a e . D u r i n g t h e n e x t 3 days a l l f i s h i n c r e a s e d t h e i r l a r v a l c a p t u r e r a t e ( F i g . 5 ) , a p p r o x i m a t i n g t h e " p l a t e a u s " r e a c h e d e a r l i e r f o r f e e d i n g on r e d Chironomus. I t appears from t h e s e e x p e r i m e n t s t h a t D o l l y Varden and c u t t h r o a t , i f s e a r c h i n g v i s u a l l y , must l e a r n t o r e c o g n i z e n o t o n l y new t y p e s o f p r e y , b u t a l s o d i f f e r e n t c o l o u r s o r s p e c i e s o f t h e same p r e y t y p e . The r e d l a r v a e were l a r g e r t h a n t h e g r e e n ones, and appeared t o move s l i g h t l y more, but t h e l a t t e r were s t i l l v e r y o b v i o u s t o t h e o b s e r v e r as t h e y l a y on t h e sand. These d a t a s u g g e s t a s e a r c h image may be i n v o l v e d , b u t t h i s a s p e c t o f l e a r n i n g was n o t i n v e s t i g a t e d f u r t h e r . 2. B e n t h i c o r i e n t e d f e e d i n g - i s o l a t e d p r e d a t o r s A. S u b s t r a t e s u r f a c e ( i ) "Exposed" p r e y O b s e r v a t i o n s a t M a r i o n Lake and t h e g i l l n e t d a t a o f Andrusak i n d i c a t e d t h a t D o l l y V a r d e n , p a r t i c u l a r l y t h e 150 -200 mm s i z e c l a s s . , f e d more h e a v i l y on b e n t h i c o r g a n i s m s t h a n d i d c u t t h r o a t . T h i s s u g g e s t e d t h a t D o l l y Varden might be more s u c c e s s f u l p r e d a t o r s t h a n c u t t h r o a t on such o r g a n i s m s . To t e s t t h i s h y p o t h e s i s an e x p e r i m e n t was c o n d u c t e d w i t h two e x p e r i e n c e d f i s h o f each s p e c i e s and Ghironomus l a r v a e as p r e y . R a t i o n s and f e e d i n g p e r i o d s were s i m i l a r t o t h o s e f o r t h e " l e a r n i n g " 24 DAY FIG. 5 Number of larvae caught (10 minute feeding session) on four consecutive days from a r a t i o n of 25 green chironomids by i n d i v i d u a l Dolly Varden and cutthroat t r o u t . 25 e x p e r i m e n t s . There was l i t t l e d i f f e r e n c e between t h e f o u r f i s h i n number o f l a r v a e c a p t u r e d ( T a b l e V I I ) b u t d i f f e r e n c e s i n s e a r c h i n g b e h a v i o r between s p e c i e s were a p p a r e n t . These w i l l be d e s c r i b e d under t h e s u b h e a d i n g " S e a r c h i n g components". TABLE V I I . F e e d i n g s u c c e s s o f i n d i v i d u a l c u t t h r o a t and D o l l y Varden s e a r c h i n g f o r 50 Chironomus l a r v a e on a sand and l e a f - l i t t e r s u b s t r a t e d u r i n g a 30-minute f e e d i n g s e s s i o n . . Number o f T r i a l s Bottom Mean g r a b s Range L a r v a e Mean c a p t u r e d Range CT #1 5 32 22-44 15 11-21 CT #2 2 61 58-64 14 13-15 DV #,1 5 67 41-85 15 10-21 DV #2 2 55 48-62 14 13-15 The l a c k o f d i f f e r e n c e i n s u c c e s s between s p e c i e s was n o t i n t e r p r e t e d t o mean t h a t t h e t r o u t were e q u a l l y e f f e c t -i v e b e n t h i c p r e d a t o r s as t h e c h a r . The e x p e r i m e n t was p r o b a b l y n o t a good t e s t o f t h i s . C h i r o n o m i d l a r v a e used were l a r g e (13 - 15 mm) and r e d i n c o l o u r . They were v e r y o b v i o u s t o a human o b s e r v e r as t h e y l a y on t h e s u b s t r a t e , and perhaps q u i t e c o n s p i c u o u s t o the. f i s h . Both s p e c i e s appeared t o s e a r c h v i s u a l l y f o r t h e s e exposed p r e y . 26 ( i i ) "Camouflaged"1 prey The combination of red Chironomus larvae on a l i g h t sandy substrate produced a s t r i k i n g colour contrast between prey and environment. Similar conditions would not l i k e l y be found i n nature. In an attempt to produce a more natural colour combination, c r y p t i c a l l y coloured (yellowish green) chironomids were used instead of Chironomus f o r the next s e r i e s . They were also smaller than Chironomus, averaging 8 - 1 0 mm. These larvae were very d i f f i c u l t to see as' they lay amongst the l e a f l i t t e r and sand. The same four predators were used, each having acquired previous experience with t h i s larva during the "learning" experiments. Feeding time was reduced to 10 minutes and the r a t i o n cut to 25 larvae. In most of the e a r l i e r feeding experiments i t was noticed that the Dolly Varden gave up searching before the end of the session, and began a type of escape behavior. Cutthroat u s u a l l y searched f o r the duration. Consequently a comparison of feeding success at the end of the period would not involve equal periods of searching. To eliminate t h i s d i f f i c u l t y searching time was recorded with a s:top watch f o r each of the species; and both prey captured and bottom-grabs were cal c u l a t e d per searching minute. F i s h were considered to be searching i f they remained within 6 - 7 cm of the bottom, and appeared to be o r i e n t i n g to i t v i s u a l l y . On t h i s basis each Dolly Varden averaged two to three 27 times as many bottom grabs, and over three times as many captures as the most e f f i c i e n t cutthroat (Table VIII)„ A Mann-Whitney U Test (Siegel 1956) showed these differences to be s t a t i s t i c a l l y s i g n i f i c a n t at the .01 l e v e l . These r e s u l t s suggest that Dolly Varden may well have some advantages over cutthroat when feeding on the benthos. Behavioral differences were again obvious. ( i i i ) Searching components The f i r s t obvious diff e r e n c e between the species concerned t h e i r pattern of swimming as they searched f o r .chironomid larvae. Gutthroat alternated between hovering i n the water column as they scanned the bottom f o r larvae, and c r u i s i n g slowly not f a r above the substrate. Dolly Varden cruis e d almost constantly when searching (Pig. 6). Their only stops i n the water column occurred o c c a s i o n a l l y when they were "chewing" and r e j e c t i n g a mouthful of sand and l i t t e r . Usually t h i s process of " s o r t i n g " and r e j e c t i n g took place while the f i s h continued to swim. They searched within 5 cm of the bottom, frequently with pectoral f i n s brushing the substrate. Cutthroat scanned the bottom from 5 to 15 cm above i t . Both species searched with bodies p a r a l l e l to, or very s l i g h t l y i n c l i n e d to the substrate, t i l t i n g the head down only to in v e s t i g a t e or pick up something. Dolly Varden usually swam more r a p i d l y than the trout as they searched f o r larvae. To quantify t h i s , the searching speeds of six d i f f e r e n t f i s h were recorded by timing t h e i r TABLE V I I I . F e e d i n g s u c c e s s (means and ranges o f 5 r e p l i c a t e s ) o f i n d i v i d u a l c u t t h r o a t and D o l l y Varden s e a r c h i n g f o r 25 g r e e n P s e c t r o t a n y p u s l a r v a e on a sand and l e a f - l i t t e r s u b s t r a t e d u r i n g a 10-minute f e e d i n g s e s s i o n . Bottom g r a b s L a r v a e c a p t u r e d M i n u t e s Bottom grabs C a p t u r e s s p e n t p e r minute p e r minute s e a r c h i n g o f s e a r c h i n g o f s e a r c h i n g Mean Range Mean Range Mean Range CT #1 2.4 0- 5 0 1 0.5-2.0 2.4 0 CT #2 18.8 7-28 2.2 0-5 8.6 6.0-9.0 2.2 .26 DV #1 19.2 15-24 2.2 0-5 2.7 1.5-3.5 7.1 .81 DV #2 34.2 12S60 5.8 1-9 6.2 4.0-8.0 5.5 .94 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 1 2 3 4 5 6 TIME IN MINUTES 6 Typ i c a l searching pattern of cutthroat and Dolly Varden looking f o r chironomid larvae i n a sand and l e a f - l i t t e r substrate. S o l i d l i n e s i n d i c a t e c r u i s i n g ; breaks i n d i c a t e hovering. Data obtained with Rustrak event recorder. 30 movement over standard 26 cm courses (20 r e p l i c a t e s each) with-i n the observation tank. Fish were searching f o r approximately 25 Chironomus on the usual sand and l e a f - l i t t e r substrate. In addition to corroborating the previously observed i n t e r m i t t e n t swimming behavior of the trout versus the constant swimming of the char, the experiments showed that the average searching speed of Dolly Varden was about twice that of cutthroat (Table IX). Differences between species were highly s i g n i f i c a n t (F = 62.58, 1, 118 d f ) . This greater swimming rate was r e f l e c t e d by Dolly Varden moving more frequently from quadrant to quadrant. There was also some i n d i c a t i o n that t h e i r searching pattern within each quadrant was more complex than that of the t r o u t . This was not q u a n t i f i e d , but might well be worth future i n v e s t i g a t i o n . TABLE IX. Average swimming speeds of i n d i v i d u a l cutthroat and Dolly Varden searching f o r Chironomus larvae on a sand and l e a f - l i t t e r substrate. F i s h Speed (cm/sec) CT #1 2.7 G.T #2 3.1 CT #3 3.0 DV #1 7.1 DV #2 5.4 DV #4 5.3 31 The r e s u l t s suggest that Dolly Varden would l i k e l y come int o contact with benthic prey more frequently than would cutthroat. Searching cutthroat and Dolly Varden seemed to have basic behavioral differences associated with picking up benthic prey. In a l l experiments with chironomids as prey both predator species seemed to respond to v i s u a l cues before picking up items from the bottom. As cutthroat scanned the bottom t h e i r eyes could frequently be seen s h i f t i n g attention from one area to another, and at times t h e i r head was also moved from side to s ide. The cutthroat's "targets" were always s p e c i f i c items such as a si n g l e l a r v a , evergreen needle, or stone. Very l i t t l e of the surrounding substrate was picked up at the same time. Do l l y Varden seemed less discriminatory as: to what they sampled. They picked up and then rejected "mouthfuls" of l i t t e r and sand, dexterously r e t a i n i n g the l a r v a while ex-p e l l i n g the sand out the front and sides of the mouth. The Dolly Varden also seemed to scan the bottom as they cruised over i t , but did not appear to v i s u a l l y s e l e c t i n d i v i d u a l items to the same extent as did the t r o u t . Cutthroat frequently spotted a target, swam over to i t , and then moved on without picking i t up. Dolly Varden on the other hand often would pick up and r e j e c t several consecutive mouthfuls of substrate with l i t t l e delay between grabs. They tended to make more bottom grabs per unit of time than the cutthroat. There was no d i s t i n c t d i v i s i o n here between species, but the i n d i v i d u a l s making fewest grabs were cutthroat and the highest rates were 32 by Dolly Varden (Table X). Probably the diffe r e n c e i n type of bottom grab i s more s i g n i f i c a n t than the diffe r e n c e i n rate of grabs. Cutthroat picked up i n d i v i d u a l items from the substrate surface while Dolly Varden engulfed mouthfuls of the substrate. On several occasions i t appeared as i f trout struck at but missed larvae. To tes t " s t r i k e success" of the two species 10 to 15 Chironomus larvae were scattered over a sand substrate a minute or two before introducing the predator. Many of the prey began burrowing i n the sand immediately and were p a r t i a l l y buried when the f i s h started to feed. Hollows-and h i l l o c k s i n the sand provided some degree of cover f o r unburied larvae. Each f i s h was tested on three or four d i f f e r e n t occasions. Strikes at larvae were scored as eithe r captures or misses. No record was kept of s t r i k e s at objects other than larvae. Cutthroat searched 5 to 15 cm above the sand and tended to dive down at larvae when sighted. They appeared to suck i n i n d i v i d u a l larvae without picking up much of the surrounding sand. Consequently they frequently missed p a r t i a l l y buried larvae or those i n small depressions i n the sand (Table XI). Dolly Varden searched within 5 cm of the bottom scooping up considerable amounts of sand with each l a r v a . Misses were rare. Three or four times a larvae was expelled with the sand, but i n each case i t was grabbed again immediately. 33 TABLE X. Number o f bottom g r a b s by i n d i v i d u a l c u t t h r o a t and D o l l y Varden d u r i n g a 10-minute f e e d i n g s e s s i o n on Chironomus l a r v a e . 1. F i f t y l a r v a e on a sand and l e a f - l i t t e r s u b s t r a t e 1 Number o f t r i a l s Number o f bottom g r a b s Mean Range CT #1 5 15 6-21 CT #2 2 30 29-32 DV #1 5 29, 16-42 DV #2 2 22 19-24 DV #6 2 49 43-56 2 2. Twenty f i v e l a r v a e on sand o n l y (no l e a f - l i t t e r ) . CT #3 3 48 41-54 CT #4 3 56 48-67 DV #3 3 56 49-66 DV #4 3 64 45-83 1 D i f f e r e n c e s between s p e c i e s were s i g n i f i c a n t a t th e .05 l e v e l (Mann-Whitney U T e s t , n 1 = 7 , n 2=9, U=7). 2 D i f f e r e n c e s were n o t s i g n i f i c a n t a t .05 l e v e l . 34 TABLE XI. S t r i k e success of i n d i v i d u a l cutthroat and Dolly Varden feeding on red Chironomus larvae on a sandy substrate. Captures Misses St r i k e success (%) CT #1 25 12 68 CT #2 45 9 83 CT #3 24 13 65 EV #1 28 0 100 DV #2 24 0 100 D;V #3 25 2 93 DV #4 31 0 100 The preceding observations suggest that Dolly Varden are more e f f e c t i v e than cutthroat i n picking up and dislodging small invertebrates p a r t i a l l y buried or i n close association with the lake bottom. The method of bottom feeding employed by Dolly Varden might also give them the p o t e n t i a l to e x p l o i t buried organisms unavailable to cutthroat. B. Food within substrate To t e s t whether Dolly Varden and cutthroat would search within the substrate i t seemed necessary to o f f e r prey i n a dense enough concentration that the predator's reward would be s u f f i c i e n t to keep i t searching. T u b i f i c i d worms were chosen f o r t h i s experiment. A l l experimental f i s h were fed Tubifex the day before the experiments s t a r t e d . Both 35 species consumed the worms immediately. On Day 1 of a 5 day serie s Tubifex were densely d i s t r i b u t e d i n the observation tank. When most worms had buried themselves thoroughly i n the 5 mm deep sand substrate a f i s h was introduced f o r a 10-minute feeding period. At the end of the period i t was removed and ingested Tubifex recovered with the stomach pump. Recovered worms were commonly broken so any piece 10 mm or longer was scored as one worm. Substrate and Tubifex were not disturbed during the serie s except f o r the addition of more worms on Day 3. Predation had l i t t l e e f f e c t on o r i g i n a l d e n s i t i e s . The four f i s h were tested i n random succession each day. Cutthroat caught only the worms that were f u l l y ex-posed on top of the sand. They picked up l i t t l e or no sand with each worm. Bottom grabs by trout were predominately at f e c a l material and a few evergreen needles present i n the sand. Dolly Varden began to s u c c e s s f u l l y e x p l o i t buried Tubifex within a few minutes of t h e i r f i r s t exposure to them (Table X I I ) . Feeding behavior consisted of slowly swimming several centimeters above and p a r a l l e l to the bottom, then t i l t i n g at an angle of 45° to 60°, and d r i v i n g and scooping i n t o the substrate. Sand grains were frequently expelled from under the operculars, and often came to res t on the broad pe c t o r a l f i n s immediately a f t e r a scoop was made. Most sand was expelled from the front and sides of the mouth as worms were "sorted" from the sand. The sorti n g of prey from substrate 36 involved a chewing movement and was very e f f e c t i v e . Worms were retained i n the mouth while v i r t u a l l y a l l of the sand was expelled. For example, DV #1 captured 5 3 Tubifex the f i r s t day but had only 20 sand grains amongst them. I t was d i f f i c u l t to determine how much the char r e l i e d on v i s u a l cues before making a grab. At times they seemed very "choosy" and appeared to look at and r e j e c t several areas before sampling Other times, p a r t i c u l a r l y a f t e r catching some worms , bottom grabs seemed undirected. TABLE XII Feeding success of i n d i v i d u a l cutthroat and Dolly Varden searching f o r Tubifex i n a sand substrate during a 10-minute feeding session. Number t r i a l s of Number of bottom qrabs Number of worms inqested Mean Range Mean Range CT #3 5 25 6-43 4 0-9 CT #4 5 26 7-48 4 0-15 DV #1 5 51 38-60 74 53-115 DV #4 5 33 20-50 36 10-65 Both species found and ate unburied Tubifex with equal r a p i d i t y when tested at the end of the 5 day experiment • (Table X I I I ) . These experiments suggest that Dolly Varden are able to e x p l o i t benthic organisms both on and within the substrate 37 whereas cutthroat are l a r g e l y l i m i t e d to prey exposed on the substrate surface. The s i z e class: of Dolly Varden used (150 - 200 mm) did not expose the tank bottom when feeding, although sand was only 5 mm deep. Perhaps i n a deeper and le s s dense substrate such as the mud of Marion Lake, organisms would be vulnerable at depths of a centimeter or more. TABLE XIII. Time taken by i n d i v i d u a l cutthroat and Dolly Varden to pick up and consume 25 Tubifex on the bottom when no substrate present. Time (min.) CT #3 5.0 CT #4 4.5 DV #1 4.0 DV #4 5.5 3. Surface oriented feeding - i s o l a t e d predators To t e s t f o r differences i n feeding a b i l i t i e s of cutthroat trout and Dolly Varden at the surface experiments were performed using Drosophila adults as prey. They were designed to t e s t whether cutthroat were quicker than Dolly Varden to f i n d and capture i n d i v i d u a l insects on the surface. Three cutthroat and two Dolly Varden were used i n the experiments. Each was tested once a day f o r four consecutive days. During the previous 60 days these f i s h had been feeding on bottom foods only (chironomid larvae and Tubifex). 38 At the s t a r t of each experiment approximately 25 f r e e z e - k i l l e d Drosophila were clumped i n one area of the observation tank. Then the experimental f i s h was introduced. The concentration of f l i e s at the surface attracted the predator's attention and caused i t to become "surface oriented". Once t h i s o r i g i n a l r a t i o n of f l i e s was consumed singl e Dros-ophila" were blown gently i n t o the tank through an a i r hose, and the length of time from a r r i v a l on the surface u n t i l capture was recorded. Immediately a f t e r each capture another f l y was introduced. This was repeated 15 times. Use of the long a i r hose permitted the observer to hide on the opposite side of the room while introducing f l i e s i n t o the tank. During these feeding sessions i t was not uncommon fo r the Dolly Varden to leave the surface area and go to the bottom. The number of t r i p s to the bottom was recorded as was the length of time of each. A sand substrate covered the bottom of the tank but there was no benthic food present. A l l f i s h quickly found and consumed the i n i t i a l concentration of f l i e s . Once cutthroat became surface oriented they cruised or hovered within 20 cm of the surface f o r most of the feeding period. Most f l i e s appeared to be sighted from no more than 15 cm away. The trout tended to be more wary than Dolly Varden of going to the surface, and often would dart up fo r a f l y , and then r a p i d l y return to a hovering p o s i t i o n 10 to 15 cm deep. When not so wary they l e i s u r e l y swam within 2 or 3 cm of the surface q u i e t l y picking up f l i e s one a f t e r another. The high average capture times f o r CT #1 and CT #4 39 on days two and three ( F i g . 7) were due to the f i s h being unusually wary of the surface. As a r e s u l t they hovered longer than usual before going f o r each f l y . Searching Dolly Varden cruised s t e a d i l y within a few centimeters of the surface. They seemed to spot f l i e s as quic k l y and from s i m i l a r distances as cutthroat, and consequently were j u s t as quick to catch f l i e s when swimming near the surface. The major diff e r e n c e between Dolly Varden and cutthroat was the propensity of the Dolly Varden to return to the substrate even when f l i e s were present. Cutthroat made no attempts to search the bottom when f l i e s were present or had recently been at the surface. Each day the char con-centrated less on the bottom, and more on the surface, thus-decreasing t h e i r average capture times (Figs. 7 and 8). 4. Surface and benthic feeding - i s o l a t e d predators The presence of prey at only one l e v e l i s u n l i k e l y to occur during the summer i n most B r i t i s h Columbia lakes. Some organisms are probably always a v a i l a b l e at both the bottom and the surface. Following the experiments i n which Dolly Varden and cutthroat were offered chironomid larvae only, t h e i r responses were examined when both chironomid larvae and surface i n s e c t s were present. E i f t y Chironomus larvae were d i s t r i b u t e d on the sand and l e a f - l i t t e r substrate as before, and i n addition 25 l i v e vestigial-winged Drosophila were scattered on the surface j u s t before introducing the f i s h . 4 0 1 1 1 " T I II III IV DAY FIG. 7 Average time i n seconds taken by Dolly Varden and cutthroat to capture s i n g l e Drosophila adults during the f i r s t four days that f l i e s were presented. 41 III IV DAY FIG, 8 Number of t r i p s to the tank bottom and averace time i n seconds of each by c u t t h r o a t and D o l l y V a r d e n during t h e i r f i r s t f o u r feeding sessions on D r o s o o h i l a a d u l t s . 42 Bottom grabs were recorded as were the number of times a f i s h l e f t the lower h a l f of the tank, and moved int o the upper h a l f (a "surface t r i p " ) . "Surface grabs" were also scored each time the f i s h nipped at something f l o a t i n g on the water, surface. When f r u i t f l i e s were present, cutthroat l e f t the bottom once they discovered the f l i e s , and exploited the surface almost completely before returning to search the bottom. U t i l i z a t i o n of the surface food supply was probably complete within 5 to 15 minutes ( F i g . 9). Cutthroat always captured 100% of the f l i e s . Once they found the f l i e s they cr u i s e d back and f o r t h within 15 cm of the surface searching f o r more. The duration of each of the f i r s t few t r i p s from the bottom to the upper h a l f of the tank was long. Sfter only one or two t r i p s each cutthroat had averaged 20 to 25 surface grabs. Dolly Varden did not stay at the surface f o r long at any one time. They o s c i l l a t e d between the surface and bottom making two to four surface t r i p s every 5 minutes, and getting one to three f l i e s per t r i p . A f t e r each b r i e f t r i p to the surface they resumed searching f o r larvae. This pattern p e r s i s t e d f o r the f u l l 30-minute feeding session, and usually r e s u l t e d i n incomplete e x p l o i t a t i o n of the f r u i t f l i e s . With f l i e s present the cutthroat made fewer bottom grabs than when f l i e s were absent, but the decrease was not s . t a t i s t i c a l l y s i g n i f i c a n t (p=.05). They did catch s i g n i f i c a n t l y 50 4 0 h DV#I PER C E N T FLIES C A U G H T IOO IOO 86 60 TIMECMINUTES) AFTER FISH INTRODUCED TO TANK Cumulative "surface grabs" by i n d i v i d u a l c u t t h r o a t and D o l l y Varden feeding on a r a t i o n of 50 Chironomus la r v a e and 25 Drosop h i l a a d u l t s . Each data p o i n t i s the mean of four r e D l i c a t e s . 44 f e w e r l a r v a e (Mann-Whitney U T e s t , p=.05). D o l l y Varden showed no s i g n i f i c a n t change i n bottom g r a b s o r c a p t u r e s ( T a b l e X I V ) . The d i f f e r e n c e s between s p e c i e s may have been more o b v i o u s i f t h e f e e d i n g p e r i o d s were s h o r t e r , i n v o l v i n g o n l y t h e t i m e when b o t h p r e y were p r e s e n t . I n t h e 30-minute s e s s i o n s c u t t h r o a t had a c h o i c e o f f l i e s o r l a r v a e f o r a p p r o x i m a t e l y 15 m i n u t e s , b u t t h e r e a f t e r o n l y l a r v a e were l e f t . T here were u s u a l l y some f l i e s p r e s e n t f o r t h e e n t i r e p e r i o d when D o l l y Varden were f e e d i n g . The c u t t h r o a t ' s p a t t e r n o f swimming changed from t h e t y p i c a l h o v e r - c r u i s e r o u t i n e t o one i n v o l v i n g a more s t e a d y p a t r o l l i n g o f t h e tan k and s c a n n i n g o f t h e s u r f a c e . No change was e v i d e n t i n t h e D o l l y Varden's swimming b e h a v i o r . The o b s e r v e d b e h a v i o r o f t h e s e f o u r f i s h s u g g e s t s t h a t c u t t h r o a t a r e q u i c k t o l e a v e t h e bottom and s l o w t o r e t u r n t o i t i f f o o d i s a v a i l a b l e on t h e s u r f a c e . D o l l y Varden seem t o be much l e s s r e a d i l y drawn t o t h e s u r f a c e , and p e r s i s t on t h e bottom as l o n g as f o o d i s p r e s e n t . 5. I n t e r a c t i v e f e e d i n g - s p e c i e s p a i r s I n M a r i o n Lake, t h e d i v i s i o n o f f o o d between D o l l y V a r d e n and c u t t h r o a t seems l a r g e l y due t o t h e l o c a t i o n o f t h e f o o d , i . e . l a k e s u r f a c e v s . l a k e b ottom. There i s some p o s s i b i l i t y t h a t t h e above r e s u l t s were b i a s e d by d i f f e r e n c e s i n f o o d p r e f e r e n c e s between s p e c i e s . Perhaps t r o u t p r e f e r r e d f r u i t f l i e s t o l a r v a e , and c h a r p r e f e r r e d t h e l a r v a e . A t t e m p t s t o r e v e a l such p r e f e r e n c e s were i n c o n c l u s i v e . To i n v e s t i g a t e TABLE XIV. C o m p a r a t i v e f e e d i n g s u c c e s s d u r i n g a 30-minute s e s s i o n o f i n d i v i d u a l c u t t h r o a t and D o l l y Varden s e a r c h i n g f o r 50 Chironomus l a r v a e i n t h e p r e s e n c e and absence o f a s u r f a c e r a t i o n o f 25 D r o s o p h i l a . F L I ES ABSENT FLIES PRESENT Number o f Number o f Number o f Number, o f Number o f Number o f t r i a l s bottom l a r v a e t r i a l s b ottom g r a b s l a r v a e g r a b s c a p t u r e d c a p t u r e d CT #1 5 32 15 6 22 10 (22-44) (11-21) (14-30) ( 3-15) CT #2 2 61 14 6 50 11 (58-64) (13-15) (38-68) ( 6-15) DV #1 5 67 15 5 51 15 (41-85) (10-21) (46-55) (14-19) DV #2 2 55 14 6 73 17 (48-62) (13-15) (28-100) (10-24) D a t a show a v e r a g e s ; ranges i n b r a c k e t s . 46 the e f f e c t of l o c a t i o n without the added complication of food preferences, experiments were made using a si n g l e food type, and paired Dolly Varden and cutthroat. T h e o r e t i c a l l y each species would ex h i b i t i t s optimum feeding area under these conditions. The experiments were of the "food scramble" type, and involved a scramble f o r food introduced item by item at the surface, the bottom, and simultaneously at both l e v e l s . Eighteen pairs of f i s h were tested but some i n d i v i d -uals were used more than once. Eleven cutthroat and 13 Dolly Varden made up these p a i r s . The same pairs were not tested-under a l l three d i f f e r e n t conditions. Pish of approximately equal lengths were paired. The food used was frozen trout f l e s h cut int o small 3 pieces about 3 mm . During each 3 to 6-day se r i e s f i s h were fed no food other than that obtained i n the experiments. Usually each p a i r of f i s h was tested i n the morning and evening, but on some days only one session was completed. Tests were repeated f i v e times f o r each p a i r . There was no substrate i n the observation tank, but a large boulder at each end provided cover f o r the f i s h . Most of the experimental f i s h were not conditioned to handling or the presence of humans. This problem was over-come by placing the paired cutthroat and Dolly Varden i n the observation tank 24 hours p r i o r to the f i r s t t e s t , and leaving them i n the tank f o r the duration of the experiment. The 47 p l e x i g l a s s windows i n the tank were covered with h a l f -aluminized p l a s t i c f i l m which acts as a one-way mirror. Food was introduced through permanently mounted p l a s t i c i n l e t pipes that protruded from both ends of the tank. A. Benthic feeding Twenty pieces of food were flushed one at a time with water down the two i n l e t pipes. The food d r i f t e d out onto the tank bottom with no obvious turbulence or bubbles to mark i t s a r r i v a l , and usually the f i s h found each piece before the next one a r r i v e d . Considerable aggression was observed i n some pairs with the r e s u l t that one f i s h usually became dominant. The number of aggressive nips was scored for three 10-minute periods before, during, and immediately af t e r feeding. These scores were used to c l a s s i f y each f i s h as the dominant or subordinate of i t s p a i r . In three of the seven pairs scrambling f o r bottom food a cutthroat dominated; no f i s h was obviously dominant i n the other p a i r s . Dominant cutthroat always got 85% or more of the r a t i o n ( F i g . 10). This was due to the intensely aggressive drives of the cutthroat quickly chasing the Dolly Varden under a rock or i n t o an upper corner of the tank. Where dominance did not occur the Dolly Varden fared much better, getting the larger share of the r a t i o n i n three of the four p a i r s . Mean transformed (angular transformation) scores of the Dolly Varden i n t h i s group were s i g n i f i c a n t l y greater than of the cutthroat 48 CUTTHROAT DOMINANT DOLLY VARDEN-DOMINANT j IOO 95 90 85 SO 75 70 65 60 55 50 45 AO 35 30 25 20 15 IO 5 O CUTTHROAT LENGTHCmnO DOLLY VARDEN LENGTH imm) CUTTHROAT WEIGHT Cg.•> DOLLY VARDEN WEIGHT Cg.) a Ui z ca O 8 z S 2 PAIR I 159 162 37 43 PAIR n 134 136 24 22 PAIR m 180 189 58 65 t PAIR EE 140 165 30 49 PAIR! 128 165 20 46 FIG. 10 Comparative success of paired Dolly Varden and cutthroat "scrambling" for 20 food items introduced one at a time at the bottom. Each data point i s the average of 5 r e p l i c a t i o n s . V e r t i c a l l i n e s i n d i c a t e range; s i z e and weight data given below. CUTTHROAT = # DOLLY VARDEN = £ 49 (t = 3.72, 3df), but variances between i n d i v i d u a l s exceeded those within (F = 3.74, 16 and 3d.f.). The species d i f f e r e d i n t h e i r manner of searching the bottom. Dolly Varden moved around quickly exploring behind the boulders and i n the corners, as well as the c e n t r a l region of the tank bottom. The cutthroat tended to alternate between hovering and slow c r u i s i n g as described previously. They stayed i n the c e n t r a l area of the aquarium, searching the corners r a r e l y . B. Surface feeding The procedure here was the same as f o r the previous experiment with the exception that food items were introduced at the surface, each making a small disturbance as i t entered the water. In four of the f i v e p a i r s cutthroat dominated, and i n these p a i r s the cutthroat never obtained less than 55% of the r a t i o n ( F i g . 11). Their mean share was s i g n i f i c a n t l y higher than the chars* (t = 8,3d.f.). The Dolly Varden was dominant i n the f i f t h p a i r , but t h i s was not unexpected since t h i s was the only grossly mismatched pair used i n the experi-ments. The char i n t h i s case weighed twice as much as the trout ( F i g . 11). Even so the cutthroat's share of the r a t i o n was always i n the 45% to 50% range. Cutthroat appeared to "out-scramble" Dolly Varden as both responded to an introduced item. Frequently there was a race f o r the b i t of food, but such races usually were won by 50 C U T T H R O A T D O M I N A N T D O M I N A N C E N O T O B V I O U S IOO 95 90 85 80 75 70 65 60 55 SO 45 40 35 30 25 20 15 IO 5 O C U T T H R O A T LENGTHOmtO DOLLY V A R D E N L E N G T H OmtO C U T T H R O A T W E I G H T Cg.) DOLLY V A R D E N W E I G H T eg.} a ui z CD o Q o o u. z UJ is! UJ 0. z 4 UJ 2 159 165 37 49 PAIR 3ZI PAIR Y E 178 172 51 50 P A I R X 159 172 37 42 128 134 19.5 25.5 156 1 6 8 29 43 135 157 23 PAIR 2 H P A I R K PAIR 2T P A I R Z E FIG. 11 Comparative success of p a i r e d D o l l y Varden and c u t t h r o a t "scrambling" f o r 20 food items int r o d u c e d one at a time at the s u r f a c e . Each data p o i n t i s the^average of 5 r e p l i c a t i o n s . V e r t i c a l l i n e s i n d i c a t e range; s i z e and weight data given below. CUTTHROAT = £ DOLLY VARDEN = ^ the t r o u t . I t was not possible to determine whether the out-come was due to trout having superior means of detecting and responding to the food, or due to a hesitance of the Dolly Varden i n the presence of a more dominant f i s h . There was more evidence of the l a t t e r than the former. Dolly Varden often l e f t the mid-water area to b r i e f l y search the bottom. The trout r a r e l y did t h i s and consequently were i n a better p o s i t i o n s p a t i a l l y to respond to the next item. C. Benthic and/or surface feeding ( i ) Observation tank. Introduction of food at the surface or at the bottom only forced the f i s h to come to the same area of the tank f o r food. To permit a s p a t i a l segregation while feeding a t h i r d experiment was run, s i m i l a r to the previous two, except that food was introduced simultaneously at the bottom and surface. The 20-itera r a t i o n was introduced two pieces at a time, one at the surface and one at the bottom of the tank. Six pairs of f i s h were tested and i n each case the cutthroat was dominant. As i n the s i t u a t i o n when surface food only was presented, the cutthroat i n each p a i r obtained an average of at lea s t 60% of the surface r a t i o n ( F i g . 12). Three of the six also got over 50% of the bottom rations as we l l . The d i f f e r e n c e i n mean scores at the bottom between Dolly Varden and cutthroat was not s i g n i f i c a n t (t = .09, 5d.f.). There was not a d i s t i n c t s p a t i a l segregation of paired f i s h i n t h i s s e r i e s . Dolly Varden, as usual, spent the 52 too 95 9 0 85 eo 75 70 65 60 55 50 45 4 0 35 30 25 20 15 IO 5 O C U T T H R O A T LENGTHCmitO DOLLY V A R D E N L E N G T H tmm) C U T T H R O A T W E I G H t i g . ; D O L L Y V A R D E N W E I G H T Cg.) D U Z CD O a O O u. O S o hJ a z 2 SURFACE BOTOM FOOD FOOD P A I R T U I 159 172 37 50 C U T T H R O A T D O M I N A N T IN A L L PAIRS SURFACE BOTOM | SURFACE BOTOM P A I R X E 180 165 58 49 PAIR Z Z 122 125 18 19 SURFACE BOTOM SURFACE BOTOM P A I R X Z L 128 138 20 28 SURFACE BOTOM PAIR X2E 118 129 16 19 PAIR w i n 125 126 22 21 F I G . 12 C o m p a r a t i v e s u c c e s s o f p a i r e d D o l l y V a r d e n and c u t t h r o a t " s c r a m b l i n g " f o r 20 f o o d i t e m s i n t r o d u c e d s i m u l t a n e o u s l y a t s u r f a c e and b o t t o m (10 s e p a r a t e i n t r o d u c t i o n s o f an i t e m a t t h e t o p and one a t t h e b o t t o m s i m u l t a n e o u s l y ) . E a c h d a t a p o i n t i s t h e a v e r a g e o f 5 r e p l i c a t i o n s . V e r t i c a l l i n e s i n d i c a t e r a n g e ; s i z e and w e i g h t d a t a g i v e n b e l o w . CUTTHROAT = 9 DOLLY VARDEN = ^ 53 majority of t h e i r time i n the lower h a l f of the tank. Much of t h i s time involved avoiding the cutthroat. Cutthroat also spent more time c r u i s i n g i n the lower than i n the upper h a l f of the tank. This p o s i t i o n was probably the most advantageous f o r spotting and responding to the surface food, which immed-i a t e l y sank. I t also put them i n p o s i t i o n to scan the bottom. In general, the cutthroat did not a c t i v e l y search the bottom. The tank was bare and they could feed o p p o r t u n i s t i c a l l y as food appeared. They seemed to be more oriented to the surface than the bottom, and responded very quickly to food coming i n overhead. Dolly Varden showed a weaker response to the surface food, but a more vigorous searching behavior on the bottom. Probably a deeper tank and f l o a t i n g surface food would have produced more s p a t i a l segregation. ( i i ) Outdoor pond In an attempt to eliminate the problems of aggression associated with the small tanks i t was o r i g i n a l l y intended to repeat the preceding experiments i n an outdoor concrete tank 2.4 x 4.5 x 0.6 m. A f t e r losing "conditioned" f i s h to i n t e r m i t t e n t but p e r s i s t e n t fungus attacks the pond was abandoned. Sbme r e s u l t s were obtained. Two Dolly Varden and two cutthroat coexisted h e a l t h i l y f o r 24 days before being removed. During t h i s period a peck order was established with the cutthroat ranking one and two i n the order. Aggressive acts were never as intense i n the pond as i n the small 54 o b s e r v a t i o n t a n k s . I f a s u b o r d i n a t e was t h r e a t e n e d i t j u s t moved o u t o f t h e way. N i p p i n g was r a r e . S u b o r d i n a t e s had t h e o p p o r t u n i t y o f a v o i d i n g dominants and did' s o . Some n i p p i n g and c h a s i n g o c c u r r e d when f i s h were c o m p e t i n g f o r a r t i f i c i a l f o o d i n t r o d u c e d one i t e m a t a t i m e . When t h e f i s h were f o r a g i n g f o r t h e c h i r o n o m i d l a r v a e and s m a l l t e r r e s t r i a l i n s e c t s p r e s e n t i n t h e pond t h e r e was v e r y l i t t l e i n t e r a c t i o n . A" D o l l y V arden was o b s e r v e d d e f e n d i n g a s e c t i o n o f t h e tank w a l l one day as i t g r a z e d on c h i r o n o m i d l a r v a e , b u t t h i s was t h e o n l y o b v i o u s example o f f o o d d e f e n s e . N e i t h e r s p e c i e s e x h i b i t e d s t r o n g t e r r i t o r i a l b e h a v i o r i n t h e pond. C u t t h r o a t were more i n c l i n e d t o have " f a v o r i t e " a r e a s , b u t u s u a l l y would t o l e r a t e a D o l l y Varden r e s t i n g w i t h i n a meter o r two. A., l a r g e p o p u l a t i o n o f s m a l l c h i r o n o m i d l a r v a e f l o u r i s h e d i n t h e t a n k , most o f them l i v i n g on t h e f o u r s i d e s . D o l l y Varden were f r e q u e n t l y o b s e r v e d " g r a z i n g " on t h e tank w a l l s , b u t c u t t h r o a t were n e v e r seen f e e d i n g i n t h i s manner. The c h a r a l s o s p e n t more t i m e a c t i v e l y s e a r c h i n g t h e sandy subs±rate w h i c h had been seeded w i t h P i s i d i u m , a few c h i r o n o m i d l a r v a e , and o t h e r i n v e r t e b r a t e s . When t h e f i s h were removed from t h e pond t h e i r stomachs were e v a c u a t e d . The c o n t e n t s a r e l i s t e d i n T a b l e XV. 6. L i g h t i n t e n s i t y and f e e d i n g To t e s t f e e d i n g a b i l i t y o f b o t h s p e c i e s a t low l i g h t i n t e n s i t i e s , i n d i v i d u a l f i s h were g i v e n 10-minute f e e d i n g s e s s i o n s a t f o u r l e v e l s r a n g i n g from 500 l u x t o 0.003 l u x a t 55 t h e s u r f a c e . No f i s h were t e s t e d more t h a n once p e r day. P r e y c o n s i s t e d o f 25 Chironomus l a r v a e e v e n l y d i s t r i b u t e d 10 m i n u t e s p r i o r t o i n t r o d u c i n g t h e f i s h . There was no s u b s t r a t e i n t h e t a n k . TABLE XV. Stomach c o n t e n t s o f D o l l y Varden and c u t t h r o a t h e l d t o g e t h e r f o r p r e v i o u s 24 days i n o u t d o o r pond. P i s i d i u m Limnaea C h i r o n o m i d l a r v a e C h i r o n o m i d pupae Winged i n s e c t s Dominant CT 0 1 2 9 3 S u b o r d i n a t e CT 0 0 2 32 63 Dominant DV 2 1 123: 3 5 S u b o r d i n a t e DV 0 1 133 70 9 P i l o t e x p e r i m e n t s w i t h two D o l l y Varden i n d i c a t e d t h a t t h e y c o u l d f e e d s u c c e s s f u l l y a t a l l o f t h e above i n t e n s i -t i e s i f g i v e n a 10-minute dark a d a p t a t i o n p e r i o d p r i o r t o f e e d i n g a t t h e 0.02 l u x and 0.003 l u x l e v e l s . A l l s i x e x p e r i m e n t a l f i s h were s u b s e q u e n t l y t e s t e d on t h i s b a s i s . A f t e r c o n s i d e r i n g r e s u l t s o f B r e t t and A l i (1958) f o r dark a d a p t a t i o n i n young s o c k e y e , I r e p e a t e d t h e t e s t s a t t h e l o w e s t l i g h t l e v e l , b u t w i t h a 40-minute dark a d a p t a t i o n p e r i o d . A t t h e maximum i l l u m i n a t i o n t e s t e d c u t t h r o a t and D o l l y Varden c a u g h t about e q u a l numbers o f l a r v a e i n t h e t a n k ( F i g . 1 3 ) . W i t h t h e f i r s t r e d u c t i o n i n l i g h t ( s u r f a c e = 0.13 l u x ) and no a d a p t a t i o n p e r i o d t h e t r o u t c a u g h t s i g n i f -56 I 3 UJ 5 U-O a. UJ CD 2 z 25-2 0 -1 5 -la-s'-A -B-(2) (21 j NO DAK ADAPTATION 1 ~ n — -515 - 2 A O I O.I3 0 0 7 C5) C3) A D V # | A DV#3 • C T # 3 • C T # I C T # 4 C T # 3 _ I O MINUTE DARK ADAPTATION jQ^*'_40 MINUTE ADAPTATION, r n r T„ M 1 — | — 0 . 0 2 o.ooe 0 . 0 0 3 O.OOI O . 0 O 3 O.OOI LIGHT INTENSITY IN LUX F I G . 13 Mean number o f l a r v a e c a u g h t f r o m a r a t i o n o f 25 C h i r o n o m u s by i n d i v i d u a l D o l l y V a r d e n and c u t t h r o a t d u r i n g a 10 m i n u t e f e e d i n g s e s s i o n . Number o f r e p l i c a t e e x p e r i m e n t s p e r f i s h shown i n b r a c k e t s ; A = w a t e r s u r f a c e l i g h t i n t e n s i t y , B = 4 8 cm be l o w s u r f a c e o r 8 cm above b o t t o m . 57 i c a n t l y fewer than the char. (Mann-Whitney U Test n-^  = 6, = 7, U a 8, p = . 0 3 7 ) . The differences became greater with the next reduction i n l i g h t . At the lowest i l l u m i n a t i o n (surface = 0.003 lux) the average captures f o r the three trout had dropped to 9% of maximum success, and char to 44% of maximum. When the dark adaptation period was lengthened to 40 minutes the success of both species increased sharply at the lowest i l l u m i n a t i o n l e v e l . With t h i s long adaptation period cutthroat reached 27% and Dolly Varden 8 3 % of the success l e v e l established at the highest l i g h t i n t e n s i t y . F i sh were also tested o c c a s i o n a l l y i n what was thought to be complete darkness. The r e s u l t s of these tests were not q u a n t i f i e d since the experimental conditions varied. In most cases neither species caught any larvae, but i n -frequently a Dolly Varden did obtain one or two. Later i t was r e a l i z e d that the room was not completely dark. Enough l i g h t f i l t e r e d i n around the covered windows to produce a reading of approximately 0.0003 lux at the tank surface. The searching behavior of both species changed at low l i g h t l e v e l s . At the lowest l e v e l the f i s h of both species searched with t h e i r snouts almost touching the bottom, and t h e i r bodies t i l t e d at angles of 45° or steeper. Cutthroat no longer hovered and scanned f o r food, but moved slowly and s t e a d i l y across: the bottom when searching. They showed a greater tendency than the Dolly Varden to give up searching 58 under these conditions, and would then swim around i n mid water, Both species appeared to move much slower than at the higher i l l u m i n a t i o n s , DISCUSSION D i s t i n c t differences i n the food habits and s p a t i a l d i s t r i b u t i o n of sympatric Dolly Varden and cutthroat were documented by Andrusak (MS 1968) for Marion Lake, where they have apparently co-existed f o r many years. The major object-i v e of the present study was to evaluate the contribution of food e x p l o i t a t i o n to the segregation of these species i n Marion Lake. In Brian's (1956) words food e x p l o i t a t i o n may occur i f one species "can f i n d and use a vacant resource more e a s i l y and quickly than the other". Hardin (1960) says " e c o l o g i c a l d i f f e r e n t i a t i o n i s the necessary condition for co-existence" and Brian (1956) means the same thing when he says "large scale co-existence i s small scale segregation". Laboratory experiments revealed a number of d i f f e r -ences i n the feeding behavior of cutthroat and Dolly Varden that might provide a means f o r e c o l o g i c a l d i f f e r e n t i a t i o n . Feeding habits seemed c l o s e l y r e l a t e d to s p a t i a l p o s i t i o n i n g . Dolly Varden rested on the bottom when not feeding and cutthroat hovered i n the water column (Table VI). When food was avail a b l e at both surface, and bottom the cutthroat exploited the surface f i r s t , but the Dolly Varden searched predominately on the benthos. During feeding experiments Dolly Varden often stopped 59 swimming t o r e s t on t h e bottom f o r a m i n u t e o r two, and f r e q u e n t l y l o c a t e d t h e i r n e x t f o o d i t e m from t h i s p o s i t i o n . Thomas ( 1 9 6 2 ) , d i s c u s s i n g t h e d i f f e r e n t f o o d s e a t e n by j u v e n i l e A t l a n t i c salmon and brown t r o u t , s p e c u l a t e s t h a t t h e t r o u t g e t more winged i n s e c t s due t o s p e n d i n g t h e i r t i m e i n m i d w a t e r . The salmon more o f t e n l i e on t h e bottom and t h e i r f o o d i s p r e d o m i n a t e l y b e n t h i c . One might ask whether f i s h e a t more b e n t h i c f o o d because t h e y r e s t on t h e bottom, o r l i e on t h e bottom because t h e y a r e f e e d i n g t h e r e . F o r D o l l y Varden t h e f o r m e r seems t o be t h e c a s e . The r e s t i n g h a b i t s o f f i s h f r o m a second l a k e , D i x o n L a k e , were t h e same as t h o s e f o r M a r i o n Lake f i s h , but i n D i x o n L a k e , where b e n t h i c o r g a n i s m s were r e l a t i v e l y s c a r c e , Andrusak (MS 1968) f o u n d t h a t c h a r f e d p r e d o m i n a t e l y on midwater and s u r f a c e f o o d s . T h e r e f o r e t h e i r r e s t i n g p o s i t i o n s o b v i o u s l y were n o t a r e s u l t o f t h e i r f e e d i n g h a b i t s . I n g e n e r a l , D o l l y Varden seem t o be a deep w a t e r f i s h compared t o c u t t h r o a t , r e g a r d l e s s o f whether t h e y a r e i n t e r -a c t i n g w i t h o t h e r s p e c i e s . D i x o n Lake c h a r a r e a l l o p a t r i c , b u t s t a y i n deep w a t e r d u r i n g t h e day and m i g r a t e v e r t i c a l l y a t dusk (A n d r u s a k , MS 1968). Perhaps t h e c h a r have a g r e a t e r p r e f e r e n c e t h a n t r o u t f o r dark a r e a s . T h i s was n o t t e s t e d i n t h e p r e s e n t s t u d y , b u t G i b s o n and K e e n l e y s i d e (1966) n o t e d t h a t young A t l a n t i c salmon s p e n t more t i m e t h a n brook t r o u t i n l i g h t e d a r e a s . They s p e c u l a t e d t h a t t h i s may be s i g n i f i c a n t i n t h e s e g r e g a t i o n o f t h e s e s y m p a t r i c s p e c i e s . 60 Horizontal d i s t r i b u t i o n and spacing of the f i s h are probably also i n t e r r e l a t e d with feeding behavior. The c h a r a c t e r i s t i c hovering by cutthroat suggests that they may take up feeding positions i n the l i t t o r a l zone. Cutthroat placed i n the outdoor pond were observed to hover i n s p e c i f i c spots, but there was l i t t l e evidence of t e r r i t o r i a l i t y . In the lake, v i s u a l observations and g i l l net data showed cutthroat to be both randomly and widely spaced. This would seem to be advantageous fo r a surface feeder i f surface food items are more dispersed than those on the bottom. In Marion Lake, d i r e c t observations and sampling with a sweep net indicated that f l o a t i n g insects were widely spaced. Dolly Varden were often contagiously d i s t r i b u t e d i n Marion Lake (Table V). There may be a d e f i n i t e f u n c t i o n a l s i g n i f i c a n c e to loose aggregates such as these when on the bottom. Keenleyside (1955) suggested that animals sometimes guide one another to food concentrations, and l i s t e d s t i c k l e -backs as an example. Benthic feeding Dolly Varden may do the same. Perhaps the combined feeding a c t i v i t i e s of a number of f i s h may s t i r up bottom organisms with the r e s u l t that each f i s h gets: more food than i f i t searched alone. S i g n i f i c a n t differences i n d i s t r i b u t i o n and spacing of the two species have been described. Their methods of searching also d i f f e r e d considerably. Dolly Varden searched more vigorously than cutthroat, and t r a v e l l e d greater distances per u n i t of time (Table IX). They tended to be more explor-atory, i n v e s t i g a t i n g the corners of the tanks, as well as 61 around and behind large rocks. This type of searching would seem well suited f o r f i s h feeding on concealed or p a r t i a l l y concealed benthic organisms. Beukema (1963) says that swim-ming a c t i v i t y i s one of the most important factors determining area searched and prey encountered per unit time. The i n d i c a -t i o n that Dolly Varden have a more complex searching pattern as well as a f a s t e r one i s i n t e r e s t i n g . Beukema (1968) d i s -cusses how a high turning rate increases the predator's e f f i c i e n c y i f prey are aggregated. Northcote (MS 1952) and others have shown that chironomid larvae are often d i s t r i b u t e d contagiously. Frequent turning by searching Dolly Varden would tend to keep the f i s h within such d i s t r i b u t i o n s . Cutthroat trout often hovered i n one place, scanning the bottom or surface u n t i l f i n d i n g a p o t e n t i a l prey. Their habit of hover-ing i n one place or a l t e r n a t e l y hovering and c r u i s i n g i s c h a r a c t e r i s t i c of many v i s u a l predators that wait f o r food to come to them. This pattern of searching would seem to be most advantageous to a piscivorous predator, but few of the trout i n Marion Lake reached the s i z e c l a s s ( > 200 mm) that fed on other f i s h . D r i f t i n g t e r r e s t r i a l i n s e c t s and caddis f l y larvae moving along the bottom are probably the most common prey. Due to differences i n searching behavior on the bottom, cutthroat have fewer benthic organisms a v a i l a b l e to them than Dolly Varden. Ricker (1954) and Tinbergen et a l . (1967) suggest that there i s a "maximum safe density" f o r prey. At t h i s and lower de n s i t i e s predators w i l l not f i n d i t rewarding to search f o r them, but predation w i l l occur at higher d e n s i t i e s . 62 In Marion Lake t h i s density i s probably higher f o r trout than f o r char. I t was c l e a r l y demonstrated i n the laboratory that cutthroat got only those organisms that were exposed on top of the substrate. The Dolly Varden's a b i l i t y to feed on buried Tubifex (Table XII) i n d i c a t e s that i t can prey on both exposed and buried organisms. In Marion Lake chironomid larvae and Pisidium were among the most important foods i n the Dolly Varden d i e t , but made an i n s i g n i f i c a n t c o n t r i b u t i on to the trout's d i e t . This held true even when surface insects were scarce i n ea r l y May and trout were feeding on the bottom ( F i g . 2). Chironomid larvae were r e a d i l y eaten by trout i n the laboratory. The absence of them i n netted trout may be l a r g e l y due to t h e i r u n a v a i l a b i l i t y to cutthroat i n the lake. Pisidium are probably equally unavailable. They also seem to be r e l a t i v e l y unpalatable to the t r o u t . In the laboratory they were always rejected when i n t a c t and s t i l l ignored i n most cases when the s h e l l s were crushed. Dolly Varden rejected them too i n most instances but occasi o n a l l y an i n d i v i d u a l would eat several i n succession. In the lake they are an important food item. Andrusak (MS 1968) found them i n one of every three Dolly Varden analyzed with up to 25 or more i n s6me& stomachs. This could be due to a r e l a t i v e s c a r c i t y of other benthic foods i n the lake. He did not f i n d a singl e Pisidium i n trout stomachs. Cutthroat trout i n the laboratory were more "surface oriented" than Dolly Varden, and responded quickly to surface i n s e c t s a f t e r weeks of feeding on benthic foods only. 63 Andrusak*s data i n d i c a t e that t h i s also occurs i n Marion Lake. On the day a f t e r the lake became i c e - f r e e , t e r r e s t r i a l insects made up 10% of the cutthroat stomach contents. Dolly Varden stomachs contained none. A. week l a t e r t h i s had r i s e n to 86% f o r the cutthroat and about 12% f o r Dolly Varden ( F i g . 2). This seems to be a c l e a r example of one species f i n d i n g and" e x p l o i t i n g a food resource before the other. On the other hand i t may j u s t r e f l e c t a r e l a t i v e lack of searching at the surface by Dolly Varden even when food i s reasonably p l e n t i f u l . In the laboratory i t was demonstrated that Dolly Varden would feed on winged i n s e c t s , but were slower than trout to switch from benthic to surface feeding, and did so only when bottom food was absent or r e l a t i v e l y scarce ( F i g . 8). They did not o r i e n t to the surface as quickly or as p e r s i s t e n t l y as cutthroat, but when they were near the surface seemed to locate i n d i v i d u a l f l i e s as r a p i d l y and from as f a r away as cutthroat. The cutthroat's habit of hovering i n the water column, and the r a p i d i t y with which i t becomes surface oriented a f t e r catching a surface prey are obviously advantageous fo r e x p l o i t i n g the surface. Andrusak (MS 1968) found that char i n Marion Lake ate large numbers of winged insects during June, and he speculated that t h i s may have been due to a temporary super-abundance of surface food. The d i f f e r e n c e i n time required and s t i m u l i necessary to switch from bottom to surface food apparently i s very s i g n i f i c a n t i n the segregation process. The r e s u l t s of experiments involving paired Dolly Varden and cutthroat were complicated by i n t e r s p e c i f i c 64 aggression, but did suggest trends i n the feeding behavior of these f i s h when they co e x i s t . When food was presented at the surface only the cutthroat c o n s i s t e n t l y "outscrambled" the Dolly Varden. Aggres-sion was a fa c t o r here, but s p a t i a l p o s i t i o n and or i e n t a t i o n were also important. Even a small subordinate cutthroat ob-tained almost 50% of the food r a t i o n ( F i g . 11). When food was introduced at the bottom cutthroat got most of i t i f they were highly dominant. I f dominance was i n doubt or the feeding area was large enough, Dolly Varden apparently got s i g n i f i c a n t l y more than the trout, probably because of a more vigorous pattern of searching the bottom. When food was presented at both l e v e l s the char seemed less d i s t r a c t e d than the trout by the surface r a t i o n . In the six pa i r s tested the Dolly Varden i n each p a i r got a greater share of the bottom food than of the surface r a t i o n . This may be l a r g e l y due to the f a c t that the cutthroat monopol-ize d the surface, leaving food a v a i l a b l e to the char at the bottom only. However, the experiments with i n d i v i d u a l Dolly Varden in d i c a t e d persistence on the benthos when food was av a i l a b l e at both l e v e l s , arid no competitors were present. Stomach analysis of the f i s h i n the outdoor pond supported both laboratory and Marion Lake f i e l d data. Although only an i s o l a t e d and non-repeated observation i t gave addition-a l evidence that when surface food i s av a i l a b l e the trout do not a c t i v e l y e x p l o i t chironomid larvae. The char on the other 65 hand feed on them extensively. The r o l e of aggression during competition f o r food has been discussed at length by Magnuson ( 1 9 6 2 ) , MacPhee ( 1 9 6 1 ) , and others. Whenever Dolly Varden and cutthroat were paired i n aquaria i n the present study aggressive i n t e r a c t i o n s r e s u l t -ed and i n many cases were intense. Cutthroat dominated i n 85% of the 25 p a i r s observed. The extreme i n t e n s i t y and frequency of aggression was thought to be mainly due to the small s i z e of the tanks. The amount of aggression as Nilsson ( 1 9 6 5 ) , MacPhee ( 1 9 6 1 ) , and Magnuson (1962) a l l reported, was corr e l a t e d with the amount of food present. Food linked t e r r i t o r i a l i t y i n streams has been report-ed by Kalleberg (1958) and Chapman ( 1 9 6 6 ) . A stream o f f e r s a v a r i e t y of habitats such as r i f f l e s and pools, with room f o r many t e r r i t o r i e s i n each. In a lake where f i s h must forage f o r food rather than wait f o r d r i f t to pass, i t i s d i f f i c u l t to imagine what advantages would r e s u l t from frequent aggressive encounters. Perhaps lake dwellers are more l i k e l y to meet other f i s h as they search f o r food, and i f so, are u n l i k e l y to e s t a b l i s h recognizable peck orders, or afford the time and energy to f i g h t with every "stranger". Chapman (1966) suggests that feeding t e r r i t o r i e s i n streams provide f o r optimal growth of i n d i v i d u a l s "with minimal wastage of energy i n unnecessary aggression". Aggression must also be minimized somehow i n lakes. Cutthroat might do t h i s by mutual avoidance i f they are not t e r r i t o r i a l . Dolly Varden are more gregarious and aggression i s probably minimal other than at spawning time. 66 The aggressive behavior of Marion Lake cutthroat and Dolly Varden d i f f e r e d completely from that of the f i s h studied by Newman (I960), He described Dolly Varden as being extremely aggressive, and cutthroat as less aggressive than other salmonid species. I t i s d i f f i c u l t to explain the c o n f l i c t i n g observa-tions of Newman and myself. Perhaps aggressive tendencies vary considerably between populations, or the differences could r e s u l t from v a r i a t i o n s i n our experimental f a c i l i t i e s . Newman speculated that the low aggression l e v e l s of dominant cutthroat "may have been r e l a t e d to poorer adjustment to aquarium conditions". This seems l i k e l y since newly caught cutthroat i n the present study took longer to adjust to the aquaria. They were sometimes dominated by Dolly Varden during t h i s period, but t h e i r p o sitions reversed l a t e r . I t i s not known at present what i n t e r a c t i o n may occur between j u v e n i l e trout and char i n Marion Lake. The exact time and l o c a t i o n of spawning has not been determined f o r either species. I t i s conceivable that sometime ea r l y i n t h e i r l i f e h i s t o r y , perhaps i n a stream environment, Marion Lake Dolly Varden might gain a " d i r e c t appreciation of the unpleasant nature of the other species" (Brian 1956). I f so, t h i s could r e i n f o r c e the s p a t i a l segregation a t t r i b u t e d to feeding d i f f e r e n c e s . Marion Lake Dolly Varden and cutthroat d i f f e r e d considerably i n the size of t h e i r heads and mouths (Fig. 1), The most s t r i k i n g feature was the small, s l i g h t l y subterminal mouth o f t h e D o l l y Varden - a s p e c i e s u s u a l l y d e s c r i b e d as p r e d a t o r y and p o s s e s s i n g a l a r g e mouth. F r e s h w a t e r f i s h c o m m u n i t i e s i n t h e n o r t h e r n h emisphere have been d e s c r i b e d by H a r t l e y (1948) as b e i n g made up o f a number o f g e n e r a l i z e d f e e d e r s , a l l f e e d i n g on t h e same f o o d s i n v a r i o u s p r o p o r t i o n s . O t h e r a u t h o r s have s t r e s s e d t h e d i f f e r e n c e s r a t h e r t h a n s i m i l a r i t i e s i n f o o d h a b i t s , and have r e l a t e d t h e s e t o mouth and body f o r m . K e a s t and Webb (1966) s t u d i e d 14 c o h a b i t i n g f i s h e s i n an O n t a r i o l a k e and c o n c l u d e d t h a t each had s p e c i f i c a d a p t a t i o n s f o r f e e d i n g on d i f f e r e n t p r e y , b u t s t i l l r e t a i n e d t h e a b i l i t y t o f e e d on a wide range o f o r g a n i s m s . N o r t h c o t e (1954) p o i n t e d o u t d i f f e r e n c e s i n mouth s t r u c t u r e o f two c o t t i d s and r e l a t e d t h e s e t o f e e d i n g h a b i t s . The mouths o f M a r i o n Lake D o l l y Varden l o o k e d much l i k e t h e i l l u s t r a t i o n o f Umbra's mouth g i v e n by K e a s t and Webb ( 1 9 6 6 ) , and d e s c r i b e d by them as a s coop. D o l l y Varden p r e y i n g on T u b i f e x were o b s e r v e d t o "scoop up" raouthfuls o f sand and worms. Perhaps t h e i r mouths a r e p a r t i c u l a r l y w e l l a d a p t e d f o r t h i s t y p e o f f e e d i n g . The c u t t h r o a t ' s mouth w i t h i t s l a r g e gape i s t y p i c a l o f an a c t i v e p r e d a t o r , w e l l s u i t e d f o r s t r i k i n g a t and g r a s p -i n g i n d i v i d u a l p r e y . V e r t e b r a t e s made o n l y a s m a l l c o n t r i b u -t i o n t o t h e stomach c o n t e n t s o f c u t t h r o a t n e t t e d d u r i n g J u l y and A'ugust by Andrusak ( F i g . 2 ) , b u t were c o m p l e t e l y a b s e n t i n t h e D o l l y V a r d e n . I n t h e l a b o r a t o r y salmon f r y were o c c a s i o n a l l y o f f e r e d t o i n d i v i d u a l f i s h . C u t t h r o a t c a p t u r e d and consumed v e r y e f f i c i e n t l y . D o l l y Varden a t t a c k e d , but a f t e r each c a p t u r e had d i f f i c u l t y i n h a n d l i n g them, and u s u a l l y 68 the f r y were rejected a f t e r being k i l l e d or c r i p p l e d . This may have r e f l e c t e d the previous experiences of i n d i v i d u a l f i s h i n Marion Lake, but also appeared to involve basic differences i n feeding adaptations and behavior. The differences i n success between Dolly Varden and cutthroat feeding at low l i g h t i n t e n s i t i e s ( F i g . 13) may be very s i g n i f i c a n t . In the laboratory, Dolly Varden fed more su c c e s s f u l l y than trout on the benthos when i l l u m i n a t i o n was adequate. I f they are also capable of feeding i n l i g h t regimes too low f o r the cutthroat t h e i r p o t e n t i a l advantages on the bottom are increased considerably. Ghapman (1966) b r i e f l y discusses the importance of v i s u a l stimulation to feeding salmonids, and states that bottom organisms must move to be attacked. This g e n e r a l i z a t i o n may hold true f o r cutthroat t r o u t , but i s less l i k e l y to apply to Dolly Varden. In the laboratory i t was not the case f o r e i t h e r species, but t h i s might have been the r e s u l t of the f i s h l e a r n -ing to recognize and to expect non-living foods. Dolly Varden caught more larvae than cutthroat at the low i n t e n s i t i e s e i t h e r because they could see better, or because they were less dependent on v i s i o n . Obviously both species r e l i e d p r i m a r i l y on v i s i o n . This was indicated by the sharp increases i n larvae captured following the longer period of dark adaptation. I t was also suggested by the infrequent captures when the tank was intended to be i n complete darkness, but a c t u a l l y had a surface i l l u m i n a t i o n of about 0.0003 lux. At t h i s i n t e n s i t y the e x t i n c t i o n l e v e l was probably not f a r o f f . Brett and Groot 69 (1963) found that young salmon no longer caught Daphnia when surface i l l u m i n a t i o n reached approximately 0.0001 lux. These prey were moving midwater forms. Girsa (1959) mentioned that gammarids were more accessible to f i s h at low l i g h t i n t e n s i t i e s i f suspended by threads i n midwater than i f on the bottom or walls of the aquarium. The Dolly Varden's superior a b i l i t y to f i n d larvae on the dark tank bottom i s believed to be due to i t s less d iscriminatory sampling behavior. Many objects varying i n shape, and form are probably tasted f i r s t , and then rejected or accepted. The cutthroat under bright l i g h t selected s p e c i f i c i n d i v i d u a l t argets. In the semi-darkness v i s u a l acuity i s s a c r i f i c e d f o r rod v i s i o n (Hoar 1966). The trout probably can no longer discern d e t a i l or s l i g h t movements of prey, and consequently pass them by. The p o s s i b i l i t y also e x i s t s that the char's night v i s i o n was superior to the t r o u t s ' . This was not tested. I n t e r a c t i v e segregation was described by Brian (1956) as the competitive method of segregation. Nilsson (1967) suggests competition and/or predation are involved. In both cases i t i s implied that i n t e r a c t i v e segregation r e s u l t s from competition between populations and i s behavioral i n nature. The question of whether or to what degree food competition occurs between f i s h populations has been considered by many. In a l l cases the conclusions have been speculative owing to the near i m p o s s i b i l i t y of measuring supply and demand 7G of resources i n the environment. Larkin (1956) discusses the subject i n depth, and s e r i o u s l y questions whether competition f o r food between populations occurs to any measurable extent. S.traskraba et a l . (1966) concluded that competition was minimal between coex i s t i n g populations of sculpins, minnows, and brown t r o u t . These species exploited d i f f e r e n t feeding areas i n a common environment. F i s h of d i f f e r e n t s i z e s may reduce competition by having d i s s i m i l a r d i e t s according to Keast (1965), and Keast and Webb (1966) emphasize that the c o e x i s t -ing species they studied each have morphological adaptations• f o r preying on d i f f e r e n t prey. Intense competition between them i s therefore u n l i k e l y . I f the segregation of Dolly Varden and cutthroat i n Marion Lake i s of the i n t e r a c t i v e type then by d e f i n i t i o n i t involves i n t e r s p e c i f i c competition. Andrusak (MS 1968) could not demonstrate, but did i n f e r that such competition was a s i g n i f i c a n t f a c t o r i n the food segregation of these species. Newman (1956) f e l t that two salmonid species "are p o t e n t i a l l y t h e i r own greatest competitors, since they have not nec e s s a r i l y evolved mechanisms f o r i n t e r s p e c i f i c t o l e r a t i o n " but are b i o l o g i c a l l y so s i m i l a r . The r e s u l t s of t h i s study suggest that Marion Lake salmonids have evolved mechanisms f o r " t o l e r a t i n g " each other. D i s t i n c t differences i n feeding adaptations and s p a t i a l preferences are involved, and these are as obvious i n i s o l a t e d i n d i v i d u a l s as i n i n t e r a c t i n g p a i r s . I t seems unnecessary to i n f e r that food competition i s la r g e l y responsible f o r the 71 segregation of the populations. Even when the species were forced to feed together on the benthos due to i c e cover i n e a r l y May ( F i g . 2) the d i e t s varied considerably. One would have expected to see some chironomid larvae present i n cutthroat stomachs at t h i s time i f competition f o r benthic foods was important to segregation. I t seems reasonable that the observed food and s p a t i a l segregations are the l o g i c a l r e s u l t s of two species e x p l o i t i n g the habitats they are best suited f o r -regardless of the presence of the other. Competition does not push them apart, but superabundance of food may bring them together. The d i f f e r e n c e i n mouth siz e between Marion Lake Dolly Varden and cutthroat may be a morphological feeding adaptation, and consideration should be given to the evolution of i t and other feeding adaptations. Thomas (1962) says that i f i n t e r s p e c i f i c competition i s the cause of these feeding adaptations the competition must have been between d i s t a n t l y r e l a t e d species during s p e c i a t i o n , or through competition of sympatric s i b l i n g species a f t e r they become g e n e t i c a l l y unique. He sees a lack of evidence f o r i n t e r s p e c i f i c competition, and concludes that physico-chemical and i n t r a s p e c i f i c factors are more important evolutionary forces. The evidence f o r food competition occurring now between Dolly Varden and cutthroat populations i n Marion Lake i s weak, but the feeding adaptations seem to have evolved as s p e c i f i c benefits towards coexistence of these species. Is i t 72 not l i k e l y that when these salmonids f i r s t shared the lake competition between them would be intense? They no doubt were b a s i c a l l y s i m i l a r and t h e i r own greatest competitors, but natural s e l e c t i o n can change gene frequencies quite r a p i d l y . The evolving populations would tend to have increasing mor-phological and behavioral differences favourable to coexistence. This accentuation of differences i n sympatric s i t u a t i o n s has been described as character displacement by Brown and Wilson (1956) and involves genetic changes i n one or more characters. The characters may be behavioral, p h y s i o l o g i c a l , e c o l o g i c a l , or morphological. Brown and Wilson say that following the i n i t i a l contact of two s i m i l a r species, the divergence of characters w i l l tend to cause the sympatric species to be more d i f f e r e n t from one another than a l l o p a t r i c ones. Kohn and Orians (1962) agreed e n t i r e l y with the above authors, and went on to stress the importance of the e f f e c t s of natural s e l e c t i o n on e c o l o g i c a l characters. These characters, although often less obvious than the morphological ones, tend to be the most important i n determining the degree of coexistence. I t i s not s u r p r i s i n g that evidence f o r food competi-t i o n between f i s h populations i s usually weak or lacking. The period of intense competition probably has ended before most i n v e s t i g a t i o n s begin. The f i e l d study by Johannes and Larkin (1961) of sympatric redside shiners (Richardsonius balteatus) and rainbow trout (Salmo qairdneri) indicated that competition f o r Gammarus was n e g l i g i b l e . Data of other workers f o r the previous 10 to 15 years revealed that at one time, shortly 73 a f t e r the i n t r o d u c t i o n of shiners, amphipods were the most important item of competition. Sympatric populations looked at may often be those that have, through natural s e l e c t i o n , evolved ways to coexist with minimal i n t e r f e r e n c e . I believe t h i s to be the case i n Marion Lake. Is the observed segrega-t i o n of Dolly Varden and cutthroat trout i n t e r a c t i v e or s e l e c t -i v e segregation? Nilsson chose the former term to avoid the ambiguities associated with the word competition, but perhaps the use of t h i s term creates as many problems as i t solves. I t i s d i f f i c u l t to decide at what point i n t e r a c t i v e segregation would become s e l e c t i v e segregation i f one continuous evolution-ary process i s involved. C e r t a i n l y i t i s an o v e r s i m p l i f i c a t i o n to think i n terms of two d i s t i n c t methods of segregation as suggested by Brian (1956). The methods both involve natural s e l e c t i o n , the d i f f e r e n c e being the degree of s e l e c t i o n attained. Perhaps the i n i t i a l tendency to segregate i s b a s i c a l l y behavioral i n nature and therefore of the i n t e r a c t i v e type, but even then the i n d i v i d u a l s from each population show-ing the greatest amount of segregation may be those best equipped g e n e t i c a l l y . The comparison of f i s h l i v i n g a l l o p a t r i c a l l y and sympatrically does not alone appear to be a s a t i s f a c t o r y approach to the study of i n t e r a c t i v e segregation. Unless the previous h i s t o r y of the sympatric populations i s well documented i t i s not v a l i d to assume that each population i s s i m i l a r i n characters to some other a l l o p a t r i c population. Character displacement, as pointed out by Brown and Wilson (1956) may 74 make them quite d i s s i m i l a r . Future tests of the i n t e r a c t i v e segregation hypothesis would be most meaningful i f they included comparative studies of the behavior and physiology of the same species from a l l o p a t r i c and sympatric s i t u a t i o n s . The introduction of species as suggested by Nilsson (1967) would be f r u i t f u l , but care should be taken i n choosing the f i s h to be introduced. The best combination would be two species whose previous h i s t o r i e s were e n t i r e l y a l l o p a t r i c . The objectives of t h i s study were to describe the feeding behavior of the two species, and to evaluate the p o t e n t i a l of food e x p l o i t a t i o n as a mechanism of i n t e r a c t i v e segregation. The hypothesis that Dolly Varden could feed more s u c c e s s f u l l y on the benthos, and cutthroat at the lake surface was supported by the r e s u l t s of laboratory experiments. The observed differences i n feeding behavior were c e r t a i n l y s u f f i c i e n t to permit e x p l o i t a t i o n by Dolly Varden at the lake bottom or cutthroat at the surface. However, there was l i t t l e evidence that food e x p l o i t a t i o n occurs to any measurable extent. E x p l o i t a t i o n involves one species f i n d i n g and using a resource before the other. I t seems u n l i k e l y that one species could "scramble away the resource i n question before the other has t r i e d to use i t " as Nilsson (1967) suggests. I would expect more overlap i n the d i e t s of trout and char i f e x p l o i t a t i o n were occurring. Each species would have some of the resource, but the more e f f i c i e n t one would have the 75 bulk of i t . I t does not appear as i f Marion Lake Dolly Varden t r y to u t i l i z e surface organisms when benthic foods are a v a i l -able, nor do the cutthroat t r y to use the same groups of benthic organisms as Dolly Varden i f surface insects are present. This study suggested that the segregation of f i s h i n Marion Lake i s not b a s i c a l l y of the i n t e r a c t i v e type postulated by N i l s s o n . I t does not deny the p o s s i b i l i t y of such segrega-t i o n , but questions whether the methods or mechanisms are t r u l y d i f f e r e n t from those of s e l e c t i v e segregation. 76 SUMMARY 1. Indivi d u a l Dolly Varden and cutthroat from the sympatric Marion Lake populations showed d i s t i n c t differences i n feeding behavior and s p a t i a l d i s t r i b u t i o n i n the laboratory. 2. Dolly Varden i n aquaria were benthic oriented and rested on the bottom. Cutthroat were often surface oriented and rested i n the water column. 3. Dolly Varden had small "scoop-like" mouths possibly well adapted f o r benthic feeding. Cutthroat mouths were large i n comparison and probably better suited f o r s e i z i n g i n d i v i d u a l moving prey. 4. The steady swimming and frequent sampling by searching Dolly Varden i s an e f f i c i e n t method of e x p l o i t i n g the benthos. The alternate hovering and c r u i s i n g of searching cutthroat i s more e f f e c t i v e f o r detecting surface and a c t i v e l y moving prey. 5. Dolly Varden were capable of e x p l o i t i n g buried food organisms unavailable to cutthroat. 6. Dolly Varden fed more s u c c e s s f u l l y than cutthroat on the benthos at low l i g h t i n t e n s i t i e s . This was believed due to them depending less on v i s i o n and more on taste than the t r o u t . 77 7. The differences i n behavior between Marion Lake Dolly Varden and cutthroat were obvious i n i s o l a t e d i n d i v i d u a l s . There was no evidence that these differences were magnified when the species were combined. 8. Behavioral and e c o l o g i c a l differences were not unequivocally r i g i d . Each species was p l a s t i c enough to change habitat and d i e t when necessary or advantageous. 9. The p o t e n t i a l f o r food e x p l o i t a t i o n existed due to species differences i n feeding adaptations, but there was no evidence that e x p l o i t a t i o n occurred to any extent. 10. The segregation of Dolly Varden and cutthroat i n Marion Lake was concluded to be the r e s u l t of basic genetic differences i n morphological, e c o l o g i c a l , behavioral, and p h y s i o l o g i c a l characters, rather than an i n t e r a c t i v e segregation as i n f e r r e d by Andrusak (MS 1968). 78 BIBLIOGRAPHY A n d r u s a k , H., 1968. 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C o n t r i b u t i o n t o t h e p r o b l e m o f f o o d c o m p e t i t i o n among t h e s c u l p i n , minnow, and brown t r o u t . J . Anim, E c o l . 3 5 ( 2 ) : 303-311. Thomas, J.D. 1962. The f o o d and growth o f brown t r o u t (Salmo  t r u t t a L.) and i t s f e e d i n g r e l a t i o n s h i p s w i t h t h e salmon p a r r (Salmo s a l a r L.) and t h e e e l ( A n g u i l l a  a n g u i l l a L.) i n t h e r i v e r T e i f y , West Wales, J . Anim. E c o l . 3 1 ( 2 ) : 175-205. T i n b e r g e n , N., M. Impekoven, and D. F r a n k . 1967. An e x p e r i m e n t on s p a c i n g - o u t as a d e f e n s e a g a i n s t p r e d a t i o n . B e h a v i o u r 28: 307-321. 

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